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TABLE OF CONTENTS
INDEX TO FINANCIAL STATEMENTS

Table of Contents

Filed Pursuant to Rule 424(b)(4)
Registration No. 333-250070
Registration No. 333-251111

PROSPECTUS

7,000,000 shares

LOGO

Sigilon Therapeutics, Inc.

Common stock

This is an initial public offering of shares of common stock of Sigilon Therapeutics, Inc. We are selling shares of our common stock. Prior to this offering, there has been no public market for our shares of common stock. The initial public offering price is $18.00 per share.

We have applied for listing of our common stock on The Nasdaq Global Select Market under the symbol "SGTX."

We are an "emerging growth company" and a "smaller reporting company" under federal securities laws and are subject to reduced public company reporting requirements. See "Prospectus Summary—Implications of Being an Emerging Growth Company and a Smaller Reporting Company."

 
 
Per share
 
Total

Initial public offering price

  $18.00   $126,000,000

Underwriting discounts and commissions(1)

  $1.26   $8,820,000

Proceeds to Sigilon Therapeutics Inc., before expenses

  $16.74   $117,180,000

(1)
See "Underwriting" for additional disclosure regarding underwriting compensation.

We have granted the underwriters an option for a period of 30 days to purchase up to 1,050,000 additional shares of common stock from us at the initial public offering price, less underwriting discounts and commissions.

Investing in our common stock involves a high degree of risk. See "Risk Factors" beginning on page 12 of this prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities, or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The underwriters expect to deliver the shares to purchasers on or about December 8, 2020.

Joint bookrunning managers

MORGAN STANLEY   JEFFERIES   BARCLAYS   CANACCORD GENUITY

   

December 3, 2020.


Table of Contents


TABLE OF CONTENTS

 
  Page  

PROSPECTUS SUMMARY

    1  

RISK FACTORS

    12  

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

    79  

USE OF PROCEEDS

    80  

DIVIDEND POLICY

    81  

CAPITALIZATION

    82  

DILUTION

    84  

SELECTED FINANCIAL DATA

    86  

MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS OF OPERATIONS

    89  

BUSINESS

    113  

MANAGEMENT

    171  

EXECUTIVE AND DIRECTOR COMPENSATION

    181  

CERTAIN RELATIONSHIPS AND RELATED PARTY TRANSACTIONS

    197  

PRINCIPAL STOCKHOLDERS

    199  

DESCRIPTION OF CAPITAL STOCK

    202  

SHARES ELIGIBLE FOR FUTURE SALE

    207  

MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES TO NON-U.S. HOLDERS OF OUR COMMON STOCK

    210  

UNDERWRITING

    214  

LEGAL MATTERS

    223  

EXPERTS

    223  

WHERE YOU CAN FIND ADDITIONAL INFORMATION

    223  

INDEX TO FINANCIAL STATEMENTS

    F-1  

        Neither we nor the underwriters have authorized anyone to provide any information other than that contained in this prospectus or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. We and the underwriters are not making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus is accurate only as of the date on the front cover of this prospectus, regardless of the time of delivery of this prospectus or any sale of our common stock. Our business, financial condition, results of operations and prospects may have changed since that date.

        For investors outside of the United States: Neither we nor the underwriters have done anything that would permit this offering or possession or distribution of this prospectus in any jurisdiction where action for that purpose is required, other than the United States. Persons outside of the United States who come into possession of this prospectus must inform themselves about, and observe any restrictions relating to, the offering of the shares of common stock and the distribution of this prospectus outside of the United States.

        Through and including December 28, 2020 (the 25th day after the date of this prospectus), all dealers effecting transactions in these securities, whether or not participating in this offering, may be required to deliver a prospectus. This delivery requirement is in addition to the obligation of dealers to deliver a prospectus when acting as underwriters and with respect to their unsold allotments or subscriptions.

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TRADEMARKS

        We use SIGILON, SLTx, AFIBROMER and SHIELDED LIVING THERAPEUTICS and other marks as trademarks in the United States and/or in other countries. This prospectus contains references to our trademarks and service marks and to those belonging to other entities. Solely for convenience, trademarks and trade names referred to in this prospectus, including logos, artwork and other visual displays, may appear without the ® or TM symbols, but such references are not intended to indicate in any way that we will not assert, to the fullest extent under applicable law, our rights or the rights of the applicable licensor to these trademarks and trade names. We do not intend our use or display of other entities' trade names, trademarks or service marks to imply a relationship with, or endorsement or sponsorship of us by, any other entity.


MARKET AND INDUSTRY DATA

        Unless otherwise indicated, information contained in this prospectus concerning our industry and the markets in which we operate, including our general expectations, market position and market opportunity, is based on our management's estimates and research, as well as industry and general publications and research and studies conducted by third parties. We believe that the information from these third-party publications, research and studies included in this prospectus is reliable. Management's estimates are derived from publicly available information, their knowledge of our industry and their assumptions based on such information and knowledge, which we believe to be reasonable. This data involves a number of assumptions and limitations which are necessarily subject to a high degree of uncertainty and risk due to a variety of factors, including those described in "Risk Factors." These and other factors could cause our future performance to differ materially from our assumptions and estimates.

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PROSPECTUS SUMMARY

        This summary highlights information included elsewhere in this prospectus. This summary does not contain all the information you should consider before investing in our common stock. You should read and consider this entire prospectus carefully, including the sections titled "Risk Factors," "Cautionary Note Regarding Forward-Looking Statements" and "Management's Discussion and Analysis of Financial Condition and Results of Operations" and our financial statements and the related notes included elsewhere in this prospectus, before making any investment decision. Unless the context otherwise requires, the terms "Sigilon," "Sigilon Therapeutics," the "Company," "we," "us" and "our" relate to Sigilon Therapeutics, Inc.

Overview

        We are a clinical-stage biotechnology company pioneering a new class of therapeutics and seeking to develop functional cures for patients with chronic diseases by providing stable and durable levels of therapeutic molecules to patients. We have developed our Shielded Living Therapeutics, or SLTx, platform, which combines advanced cell engineering with cutting-edge innovations in biocompatible materials and enables our product candidates to produce a wide range of therapeutic molecules that may be missing or deficient, such as proteins, antibodies and hormones. We are designing our product candidates to be off-the-shelf, durable, controllable and redosable, without requiring modification of the patient's genes or immunosuppression. Our lead product candidate, SIG-001, is designed to prevent bleeding episodes in patients with moderate-severe to severe Hemophilia A by continuously secreting human FVIII. We received acceptance of our IND submission in the United States in August 2020 and our CTA in the United Kingdom in May 2020. We have initiated our Phase 1/2 clinical study of SIG-001 in Hemophilia A, with the first patient dosed in October 2020.

        Our SLTx platform is comprised of two primary elements: the cells and the sphere.

    The Cells.  Our current allogeneic human cell line was selected for its safety, durability, scalability and engineerability, and has been extensively tested in preclinical and clinical settings. This profile allows us to stably integrate a high expressing transgene, with the ability to create a self-renewing tissue to enhance durability and manufacturing scalability. The same parental cell bank is currently being used for our internal pipeline programs.

    The Sphere.  We encapsulate the cells in our proprietary biocompatible matrix, formatted as 1.5 mm spheres, consisting of (i) an inner hydrogel compartment optimized for cell viability and productivity and (ii) an outer immune-shielding hydrogel layer composed of Afibromer, an alginate conjugated with a novel, proprietary anti-fibrotic small molecule.

GRAPHIC

        Modularity, a key attribute of our SLTx platform, is comprised of three pillars: the cells, the sphere and the manufacturing process. In addition to the cells and the sphere described above, we have also spent

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significant time and resources over the last three years to create a state-of-the-art manufacturing platform that is modular for all potential product candidates developed using our cell and sphere components. This cost-effective manufacturing platform is designed to provide a true "off-the-shelf" product for patients. Furthermore, virtually all aspects of the platform are shared across our development programs, enabling a potentially streamlined path from discovery to clinical trials. With our modular platform, the only significant change among our internal product candidates is the expression cassette used in the cells, which we customize to express the desired therapeutic molecule. This modularity has created an efficient engine for generation of product candidates, allowing us to build a diverse pipeline.

        Our SLTx platform is designed to significantly improve the management of chronic diseases by overcoming the significant limitations of cell and gene therapies and the drawbacks of current standard of care biologic-based therapies. Cell therapy and viral gene therapy have been used to replace or repair missing or defective cells or genes and continue to be evaluated as potential therapeutic interventions. Despite major improvements in the field, there are still a number of limitations that spurn activity and applicability of both cell and gene therapy, including immune rejection, requirement for immune suppression, limited eligibility, durability and variability challenges, inability to re-dose and high manufacturing costs. In contrast, our SLTx platform is designed to generate product candidates with the following advantages:

    Functional Cure.  A single dose of our SLTx product candidates may provide a meaningful long-term clinical benefit to patients, as well as significant health economic advantages.

    Controllable Dosing.  We have shown in preclinical studies that various doses of SLTx product candidates can deliver predictable protein levels because of consistent expression of the therapeutic molecule by the engineered clonal cells.

    Redosable.  In preclinical studies, we have demonstrated that doses of our investigational products can be administered repeatedly, which we believe further differentiates our cell-based approach from other modalities such as gene therapy.

    Retrievable.  While we do not anticipate a need for retrieval of our product candidates from a patient, we have demonstrated in non-human primates that we can retrieve the vast majority of the spheres if necessary.

    Off-the-Shelf.  Our SLTx product candidates are designed to be off-the-shelf, allogeneic, encapsulated cell therapies.

    No Integration with the Host Genome.  Our cells are engineered outside the body and have known integration sites for the transgene. Upon placement, our cells remain inside of the spheres and do not interact with the host genome. This feature is designed to avoid safety issues potentially caused by insertions into the host genome.

    Avoidance of Pre-existing Immunity and Immune Suppression.  Our platform is non-viral cell-based gene therapy and non-immunogenic. Therefore, our product candidates are not affected by pre-existing antibodies. In addition, our product candidates do not require immune suppression or bone marrow conditioning agents.

Our Product Candidates

        Leveraging the modularity of our platform and our scientific and preclinical work to date, we are able to advance programs in distinct therapeutic areas, including rare blood disorders, lysosomal storage diseases and endocrine and other chronic disorders. We are applying a strategic sequencing to the development of our portfolio, focusing on commercial potential, unmet need, opportunity to provide meaningful clinical benefit to patients, speed to proof-of-concept, clear regulatory path and

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easy-to-measure validated protein therapeutics and clinical endpoints. Our current pipeline of SLTx product candidates is summarized in the figure below.

GRAPHIC

        SIG-001 is our most advanced product candidate, which is designed to prevent bleeding episodes in patients with moderate-severe to severe Hemophilia A. Unlike commercially available recombinant Factor VIII, or FVIII, indicated for temporary replacement of FVIII, which requires life-long repeat intravenous administrations, SIG-001 would be administered intraperitoneally and we expect each dose to have a duration of three to five years. Following administration through a short laparoscopic procedure into the peritoneal cavity, SIG-001 is designed to continuously secrete human FVIII protein that subsequently diffuses into the bloodstream, thereby eliminating the need for frequent administration of recombinant FVIII in these patients. We believe this approach has the potential to provide sustained, durable levels of FVIII and potentially improve long-term outcomes.

        Our preclinical work for SIG-001 focused on the feasibility of the proposed approach, optimization of the product, the pharmacological and pharmacodynamic profile of released human beta-domain deleted FVIII and key safety aspects in various animal species. In in vitro and in vivo models, we demonstrated that SIG-001 had dose-dependent, durable levels of FVIII and no safety or toxicology signals were identified. We received acceptance of our IND submission in the United States in August 2020 and CTA in the United Kingdom in May 2020. We have initiated our Phase 1/2 clinical study of SIG-001 in Hemophilia A, with the first patient dosed in October 2020.

        The first patient in the study, suffering from severe Hemophilia A, was dosed in October 2020 in the United Kingdom at the 50 mL dose level. During the perioperative period and the following approximately four weeks, there have been no serious adverse events. One adverse event of pain was reported after the laparoscopic procedure and has been fully resolved. We have measured FVIII activity levels using one-stage clotting assays and chromogenic substrate assays on a weekly basis for the four weeks after dosing. We have observed FVIII activity levels in the low single digits in both assays. Because severe Hemophilia A patients generally produce non-measurable FVIII activity levels, we believe these initial human data demonstrate that the cells residing in our spheres are producing measurable FVIII in this first patient at the lowest dose.

        Moreover, we are extending our reach within rare blood disorders. We are developing SIG-009 for patients with Factor VII deficiency and SIG-003 for patients with Hemophilia B.

        SIG-005 is our product candidate that contains a cell line genetically modified with a non-viral vector to express human a-L-iduronidase, or IDUA, encapsulated within our spheres. SIG-005 is being developed to treat the non-neurological manifestations of the disease in patients with mucopolysaccharidosis type 1, or MPS-1. We have completed pre-IND and scientific advisory meetings with both the U.S. Food and Drug Administration, or the FDA, and Medicines and Health product Regulatory Agency, or MHRA.

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        We believe our SLTx platform has significant applicability to treat a broad range of other lysosomal diseases. We are developing SIG-007 for patients with Fabry disease and SIG-018 for patients with mucopolysaccharidosis type 2, or MPS-2.

        SIG-002 is our product candidate designed to replace islet cells for the treatment of Type 1 Diabetes, or T1D. In T1D, the immune system attacks and destroys the insulin-producing beta cells within the endocrine islets of the pancreas. Insulin deficiency results in dysregulation of glucose metabolism. In April 2018, we partnered with Eli Lilly and Company, or Lilly, to develop cell therapies for the treatment of T1D, including SIG-002. Under the terms of the partnership, Sigilon is leading execution of the program through IND, and Lilly, a global leader in diabetes, will develop and commercialize the program worldwide. We received an upfront payment of $62.5 million as well as a $13.1 million equity investment from Lilly, and we are eligible to receive up to $165.0 million in regulatory milestones and $250.0 million in sales-based milestones and tiered, from mid-single-to-low-double digit, sales-based royalties on sales. In 2019, Lilly invested an additional $12.0 million as part of our Series B financing.

        We intend to apply the modularity of our SLTx platform to develop more product candidates and explore delivery of different molecules and alternative routes of administration. We are developing SIG-014 for patients with wet age-related macular degeneration and SIG-015 for patients with immune mediated diseases.

Strategy

        Our goal is to provide functional cures to patients with chronic diseases by applying our SLTx platform to discover, develop, manufacture and commercialize a new class of medicines. To achieve this vision and maximize value to stakeholders, we are executing a strategy with the following key elements:

    Leverage our Modular Platform.  We have focused our efforts on three pillars of our modular platform: (i) optimization of our engineered cell lines; (ii) optimization of our spheres; and (iii) development of scalable manufacturing processes. By leveraging these components, we believe that this strategy will enable us to rapidly and simultaneously pursue numerous product candidates in a capital-efficient manner.

    Focused Indication Prioritization.  Given the breadth of the applicability of our platform, we prioritize product candidates based on the potential to provide meaningful clinical benefit to patients, rapid time to proof-of-concept, clear regulatory path and validated biology and clinical endpoints. Based on this prioritization, we have focused our efforts in rare blood diseases and LSDs, in addition to our partnership with Lilly in T1D. As our platform is de-risked, we expect to target therapeutic areas and chronic diseases of increased complexity and prevalence.

    Further Strengthen our Differentiated, Proprietary and Cost-Efficient Manufacturing Capability. We have designed our manufacturing processes for reproducibility and speed. In addition, we have scaled our process for our planned Phase 1/2 studies. We expect to continue to invest in our manufacturing platform and leverage our modularity as we further scale-up our proprietary processes. We believe that, at commercial scale, our cost of goods will be similar to monoclonal antibodies.

    Driving Innovation with our Strong Patient-first Culture.  Since our formation, we have established a highly collaborative, patient-first culture that drives our passion for innovation. Our patient-first culture has allowed us to understand the needs of patients and their caregivers, including the need for innovative medicines. We have leveraged our commitment to science and our location to attract scientific talent and experienced leaders, which underlies our commitment to the patient communities we serve. In addition, we have assembled a board and scientific advisors with deep expertise in research, development, regulatory affairs and manufacturing across the therapeutic areas that we are initially targeting, and that are guided by the needs of the patient community.

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    Maximizing Value Creation.  We have assembled a management team with experience in the development and commercialization of therapeutics globally, particularly in rare diseases, and can build our own worldwide development, marketing and sales organization due to the limited commercial infrastructure required to serve rare disease markets. In other more prevalent chronic diseases, we believe that executing targeted strategic partnerships for select indications or regions of the world would be beneficial to expand and accelerate access of our technology to broader patient populations worldwide, as demonstrated by our partnership with Lilly.

Company Founding

        We were founded in 2015 by Flagship Pioneering, working together with academic co-founders Drs. Robert Langer and Daniel Anderson of MIT, to develop and commercialize a new category of therapeutics to treat human diseases. Our platform technology was inspired by a decade of work at MIT demonstrating, in principle, that capsules made of novel engineered biomaterials, which do not trigger a foreign body response (or scarring), could be implanted in animals for extended periods and support the survival of cells producing a therapeutic protein without the need for immunosuppression. A Flagship Labs innovation team at Flagship Pioneering, led by Managing Partner Dr. Douglas Cole, M.D. (Sigilon's founding and current Chairman), and, subsequently, Sigilon's research and development team, built on this seminal work to expand and scale this approach and show its potential to address a range of unmet needs in multiple therapeutic areas. Since our formation, we have established a highly collaborative, patient-first culture that drives our passion for innovation. Our management team has extensive expertise in chronic diseases, human genetics and cell and gene engineering. We are led by Dr. Rogerio Vivaldi Coelho, our President and Chief Executive Officer, who has more than 30 years of experience as a physician and as an industry executive. Prior to joining Sigilon, Dr. Vivaldi served as Executive Vice President and Chief Global Therapeutics Officer at Bioverativ Inc. from 2016 until it was acquired by Sanofi S.A. in 2018, and served as Chief Commercial Officer at Spark Therapeutics, Inc. between 2014 and 2016. Before that, he led Genzyme's rare disease business as President of both the rare disease business and the renal & endocrine group, as well as Senior Vice President and General Manager of Genzyme's Latin America Group during his 20-year tenure at Genzyme.

Pre-IPO Financing

        To date we have raised over $225 million from investors, lenders and other sources including Flagship Pioneering, CPPIB, Blackrock, Oxford, Vulcan, Sphera, QIA, Monashee and Lilly. Most recently, in October 2020, we entered into a stock purchase agreement, pursuant to which we sold 3,550,000 shares of our Series B-1 convertible preferred stock at a price of $7.00 per share to certain investors for a total of $24.9 million. See "—Certain Relationships and Related Person Transactions."

Risks Associated With Our Business

        Our business is subject to a number of risks of which you should be aware before making an investment decision. These risks are discussed more fully in the "Risk Factors" section of this prospectus immediately following this prospectus summary. These risks include the following:

    We have incurred significant losses since inception. We expect to incur losses for the foreseeable future and may never achieve or maintain profitability.

    We will need substantial additional funding. If we are unable to raise capital when needed, we would be forced to delay, reduce or eliminate our research and product development programs or future commercialization efforts.

    Management has concluded that there is substantial doubt about our ability to continue as a going concern.

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    The SLTx platform consists of novel technologies that are not yet clinically validated for human therapeutic use. The approaches we are taking to discover and develop novel therapeutics are unproven and may never lead to marketable products.

    We may not be successful in our efforts to identify and develop product candidates. If these efforts are unsuccessful, we may never become a commercial stage company or generate any revenues.

    We are early in our development efforts. It will be many years before we or our collaborators commercialize a product candidate, if ever.

    We currently have only one clinical stage product candidate, SIG-001, for which we dosed our first patient in October 2020. A failure of this product candidate in clinical development would adversely affect our business and may require us to discontinue development of other product candidates, which are all based on the same SLTx platform.

    We only have preliminary data from the first patient dosed with SIG-001 and no results from our product candidates in clinical trials and any favorable preclinical results are not predictive of results that may be observed in clinical trials.

    If any of the product candidates we may develop, or the delivery modes we rely on to administer them, cause serious adverse events, undesirable side effects or unexpected characteristics, such events, side effects or characteristics could delay or prevent regulatory approval of the product candidates, limit the commercial potential or result in significant negative consequences following any potential marketing approval.

    If we are unable to obtain and maintain patent and other intellectual property protection for SIG-001 or any other product candidates and for our SLTx platform, or if the scope of the patent and other intellectual property protection obtained is not sufficiently broad, our SLTx platform may be adversely affected.

    A pandemic, epidemic, or outbreak of an infectious disease, such the COVID-19 pandemic, may materially and adversely affect our business and our financial results and could cause a disruption to the development or supply of SIG-001 or any other product candidates.

        The foregoing is only a summary of some of our risks. For a more detailed discussion of these and other risks you should consider before making an investment in our common stock, see "Risk Factors."

Implications of Being an Emerging Growth Company and a Smaller Reporting Company

        We qualify as an "emerging growth company" as defined in the Jumpstart Our Business Startups Act of 2012, or the JOBS Act. As an emerging growth company, we may take advantage of specified reduced disclosure and other requirements that are otherwise applicable generally to public companies, including reduced disclosure about our executive compensation arrangements, exemption from the requirements to hold non-binding advisory votes on executive compensation and golden parachute payments and exemption from the auditor attestation requirement in the assessment of our internal control over financial reporting.

        We may take advantage of these exemptions until the last day of the fiscal year following the fifth anniversary of this offering or such earlier time that we are no longer an emerging growth company. We would cease to be an emerging growth company earlier if we have $1.07 billion or more in annual revenue, we have $700.0 million or more in market value of our stock held by non-affiliates (and we have been a public company for at least 12 months and have filed one annual report on Form 10-K) or we issue more than $1.0 billion of non-convertible debt securities over a three-year period. For so long as we remain an emerging growth company, we are permitted, and intend, to rely on exemptions from certain disclosure requirements that are applicable to other public companies that are not emerging growth companies. We may choose to take advantage of some, but not all, of the available exemptions.

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        We have elected to take advantage of certain of the reduced disclosure obligations in the registration statement of which this prospectus is a part and may elect to take advantage of other reduced reporting requirements in future filings. In particular, in this prospectus, we have provided only two years of audited financial statements and have not included all of the executive compensation related information that would be required if we were not an emerging growth company. As a result, the information that we provide to our stockholders may be different than you might receive from other public reporting companies in which you hold equity interests.

        In addition, the JOBS Act provides that an emerging growth company can take advantage of an extended transition period for complying with new or revised accounting standards. This allows an emerging growth company to delay the adoption of certain accounting standards until those standards would otherwise apply to private companies. We have elected not to "opt out" of such extended transition period, which means that when a standard is issued or revised and it has different application dates for public or private companies, we will adopt the new or revised standard at the time private companies adopt the new or revised standard and will do so until such time that we either (i) irrevocably elect to "opt out" of such extended transition period or (ii) no longer qualify as an emerging growth company. We may choose to early adopt any new or revised accounting standards whenever such early adoption is permitted for private companies. Therefore, the reported results of operations contained in our financial statements may not be directly comparable to those of other public companies.

        We are also a "smaller reporting company," meaning that the market value of our stock held by non-affiliates plus the proposed aggregate amount of gross proceeds to us as a result of this offering is less than $700 million and our annual revenue is less than $100 million during the most recently completed fiscal year. We may continue to be a smaller reporting company after this offering if either (i) the market value of our stock held by non-affiliates is less than $250 million or (ii) our annual revenue is less than $100 million during the most recently completed fiscal year and the market value of our stock held by non-affiliates is less than $700 million. If we are a smaller reporting company at the time we cease to be an emerging growth company, we may continue to rely on exemptions from certain disclosure requirements that are available to smaller reporting companies. Specifically, as a smaller reporting company we may choose to present only the two most recent fiscal years of audited financial statements in our Annual Report on Form 10-K and, similar to emerging growth companies, smaller reporting companies have reduced disclosure obligations regarding executive compensation.

Our Corporate Information

        We were incorporated in Delaware in May 2015 under the name VL36, Inc. and commenced operations in February 2016, when we changed our name to Sigilon, Inc. In June 2017, we changed our name to Sigilon Therapeutics, Inc. Our principal executive offices are located at 100 Binney Street, Suite 600, Cambridge, MA 02142, and our telephone number is (617) 336-7540. Our website is www.sigilon.com. Information contained on, or that can be accessed through, our website is not part of this prospectus.

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THE OFFERING

Common stock offered by us

  7,000,000 shares.

Common stock to be outstanding after this offering

 

30,190,271 shares (31,240,271 shares if the underwriters exercise their option to purchase additional shares in full).

Underwriters' option to purchase additional shares of common stock from us

 

We have granted the underwriters an option to purchase up to an aggregate of 1,050,000 additional shares of common stock from us at the initial public offering price, less the estimated underwriting discounts and commissions, for a period of 30 days after the date of this prospectus.

Use of proceeds

 

We estimate that our net proceeds from the sale of our common stock in this offering will be approximately $114.0 million, based on an initial public offering price of $18.00 per share, and after deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us.

 

We intend to use the net proceeds from this offering, together with cash on hand, for our Phase 1/2 clinical trials and ongoing development for SIG-001 for treatment of Hemophilia A, IND-enabling studies and the potential initiation of clinical studies for certain of our other current programs, to continue to scale our GMP manufacturing processes for our lead product candidates SIG-001 and SIG-005, continued advancement of our platform technologies and discovery-stage research for other potential programs and general corporate purposes. See "Use of Proceeds."

Dividend policy

 

We do not anticipate declaring or paying any cash dividends on our capital stock in the foreseeable future. See "Dividend Policy."

Risk factors

 

You should carefully read the "Risk Factors" section of this prospectus and the other information included in this prospectus for a discussion of factors that you should consider before deciding to invest in our common stock.

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Directed Share Program

 

At our request, the underwriters have reserved up to 5% of the shares of common stock offered hereby to offer, at the initial public offering price, to directors, officers, employees, business associates and related persons of Sigilon. The number of shares of common stock available for sale to the general public will be reduced to the extent these individuals purchase such reserved shares. Any reserved shares that are not so purchased will be offered by the underwriters to the general public on the same basis as the other shares offered by this prospectus. Except for any shares acquired by our directors, officers and employees, shares purchased pursuant to the directed share program will not be subject to lock-up agreements with the underwriters. See "Underwriting—Directed Share Program."

Nasdaq Global Select Market symbol

 

SGTX

        The number of shares of our common stock to be outstanding after this offering is based on 5,463,170 shares of our common stock outstanding as of October 31, 2020, and gives effect to the conversion of 39,886,001 shares of convertible preferred stock outstanding as of October 31, 2020, including 3,550,000 shares of Series B-1 convertible preferred stock issued in October 2020. These amounts exclude:

    67,191 shares of common stock issuable upon the exercise of outstanding warrants to purchase preferred stock as of October 31, 2020 that will become warrants to purchase shares of common stock at a weighted-average exercise price of $8.56 per share upon the closing of this offering;

    3,972,640 shares of common stock issuable upon the exercise of stock options outstanding as of October 31, 2020 under our Sigilon Therapeutics, Inc. 2016 Equity Incentive Plan, or the 2016 Plan, at a weighted-average exercise price of $4.38 per share;

    364,060 additional shares of common stock available for future issuance as of October 31, 2020 under our 2016 Plan;

    1,410,008 shares of common stock reserved for issuance under our 2020 Equity Incentive Plan, or the 2020 Plan, which will become effective in connection with this offering; and

    89,992 shares of common stock issuable upon the exercise of stock options granted pursuant to the 2020 Plan, which grants will become effective in connection with this offering.

        Unless otherwise noted, the information in this prospectus assumes:

    a 1-for-2.25 reverse stock split effected on November 25, 2020;

    the automatic conversion of all outstanding shares of our convertible preferred stock into an aggregate of 17,727,101 shares of common stock upon the closing of this offering, which includes 1,577,774 shares of common stock issuable upon the conversion of shares of our Series B-1 convertible preferred stock issued in October 2020;

    no exercise of the outstanding stock options or warrants described above;

    no issuance of options or warrants on or after October 31, 2020;

    no exercise by the underwriters of their option to purchase additional shares; and

    the filing and effectiveness of our restated articles of organization and the adoption of our amended and restated bylaws upon the closing of this offering.

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SUMMARY FINANCIAL DATA

        You should read the following summary financial data together with the sections titled "Selected Financial Data" and "Management's Discussion and Analysis of Financial Condition and Results of Operations" of this prospectus and our financial statements and the related notes included elsewhere in this prospectus. The statement of operations and comprehensive loss data for the years ended December 31, 2018 and 2019 have been derived from our audited financial statements included elsewhere in this prospectus. The statement of operations and comprehensive loss data for the nine months ended September 30, 2019 and 2020 and our balance sheet data as of September 30, 2020 have been derived from our unaudited financial statements included elsewhere in this prospectus. In the opinion of management, the unaudited financial data reflect all adjustments, consisting only of normal recurring adjustments, necessary for a fair statement of the financial information in those statements. Our historical results are not necessarily indicative of the results that may be expected in the future.

 
  Year Ended December 31,   Nine Months Ended September 30,  
 
  2018   2019   2019   2020  
 
  (in thousands, except share and per share amounts)
 

Statement of Operations and Comprehensive Loss Data:

                         

Revenue:

                         

Collaboration revenue from related party

  $ 4,637   $ 14,155   $ 11,057   $ 9,618  

Operating expenses:

                         

Research and development (inclusive of related party payments to MIT of $2,250, $411, $286 and $0 for the years ended December 31, 2018 and 2019 and the nine months ended September 30, 2019 and 2020, respectively)

    21,039     48,108     33,094     39,151  

General and administrative

    6,673     10,170     7,270     9,023  

Total operating expenses

    27,712     58,278     40,364     48,174  

Loss from operations

    (23,075 )   (44,123 )   (29,307 )   (38,556 )

Other income (expense):

                         

Interest income

    698     1,058     781     268  

Interest expense

    (289 )   (650 )   (462 )   (697 )

Other expense

    (81 )   (6 )   (6 )   (47 )

Change in fair value of preferred stock warrant liability

    (18 )   (204 )   (202 )   (44 )

Loss on extinguishment of debt

                    343  

Total other income (expense), net

    310     198     111     (863 )

Net loss and comprehensive loss

  $ (22,765 ) $ (43,925 ) $ (29,196 ) $ (39,419 )

Accretion of beneficial conversion feature

    (1,292 )            

Accretion of Series A convertible preferred stock to redemption value

    (1,698 )       (1,668 )    

Net loss attributable to common stockholders

  $ (25,755 ) $ (43,925 ) $ (30,864 ) $ (39,419 )

Net loss per share attributable to common stockholders—basic and diluted(1)

  $ (9.19 ) $ (10.74 ) $ (7.90 ) $ (7.56 )

Weighted average common stock outstanding—basic and diluted(1)

    2,802,634     4,090,691     3,904,805     5,214,818  

Pro forma net loss per share attributable to common stockholders—basic and diluted (unaudited)(1)

        $ (2.78 )       $ (1.87 )

Pro forma weighted average common stock outstanding—basic and diluted (unaudited)(1)

          15,703,464           21,042,984  

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  September 30, 2020  
 
  ACTUAL   PRO FORMA(3)   PRO FORMA
AS ADJUSTED(4)
 
 
  (in thousands)
 

Balance Sheet Data:

                   

Cash

  $ 62,642   $ 87,492   $ 201,617  

Working capital(2)

    18,372     43,222     158,973  

Total assets

    77,453     102,303     214,802  

Long-term debt, net of discount, including current portion

    19,740     19,740     19,740  

Preferred stock warrant liability

    446          

Convertible preferred stock

    117,149          

Total stockholders' equity (deficit)

    (114,920 )   27,525     141,650  

(1)
See Note 15 to our financial statements included elsewhere in this prospectus for details on the calculation of basic and diluted net loss per share attributable to common stockholders and unaudited pro forma basic and diluted net loss per share attributable to common stockholders.

(2)
We define working capital as current assets less current liabilities.

(3)
The pro forma balance sheet data reflects (i) the issuance and sale of 3,550,000 shares of Series B-1 convertible preferred stock in October 2020, (ii) the automatic conversion of all outstanding shares of our convertible preferred stock into an aggregate of 17,727,101 shares of common stock upon the closing of this offering, which includes 1,577,774 shares of common stock issuable upon the conversion of shares of our Series B-1 convertible preferred stock issued in October 2020, and (iii) all outstanding warrants to purchase shares of convertible preferred stock becoming warrants to purchase shares of common stock.

(4)
The pro forma as adjusted balance sheet data give further effect to our issuance and sale of            shares of our common stock in this offering at the initial public offering price of $18.00 per share, after deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us.

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RISK FACTORS

        Investing in our common stock involves a high degree of risk. You should carefully consider the risks and uncertainties described below together with all of the other information contained in this prospectus, including our financial statements and related notes appearing at the end of this prospectus, before deciding to invest in our common stock. If any of the events or developments described below were to occur, our business, prospects, operating results and financial condition could suffer materially, the trading price of our common stock could decline, and you could lose all or part of your investment. The risks and uncertainties described below are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently believe to be immaterial may also adversely affect our business.

Risks Related to Our Financial Position and Need for Additional Capital

We have incurred significant losses since inception. We expect to incur losses for the foreseeable future and may never achieve or maintain profitability.

        We are currently advancing our pipeline of programs in development. Discovering development candidates and developing investigational therapeutics is expensive, and we expect to continue to spend substantial amounts to (i) perform basic research, perform preclinical studies, and conduct clinical trials of our current and future programs, (ii) continue to develop and expand our Shielded Living Therapeutics, or SLTx, platform and infrastructure and supply preclinical studies and clinical trials with appropriate grade materials, including current good manufacturing practices, or cGMP, materials, (iii) seek regulatory approvals for our product candidates, and (iv) launch and commercialize any product candidates for which we receive regulatory approval.

        Since inception, we have incurred significant operating losses. Our net loss was $22.8 million for the year ended December 31, 2018, $43.9 million for the year ended December 31, 2019 and $39.4 million for the nine-month period ended September 30, 2020. As of September 30, 2020, we had an accumulated deficit of $120.7 million. We have financed our operations primarily through private placements of our preferred stock, payments received under our collaboration agreement and proceeds from borrowings under our credit facilities. We have devoted all of our efforts to research and development. We expect to continue to incur significant expenses and increasing operating losses for the foreseeable future. The net losses we incur may fluctuate significantly from quarter to quarter. We anticipate that our expenses will increase substantially if and as we:

    initiate clinical trials for SIG-001 or any other product candidates;

    continue our current research programs and our preclinical development of product candidates from our current research programs;

    seek to identify additional research programs and additional product candidates;

    comply with regulatory requirements established by the U.S. Food and Drug Administration, or FDA, the European Medicines Agency, or EMA, and other comparable foreign regulatory authorities;

    conduct preclinical studies for our product candidates;

    seek marketing approvals for any of our product candidates that successfully complete clinical trials;

    ultimately establish a sales, marketing, and distribution infrastructure to commercialize any medicines for which we may obtain marketing approval;

    further develop our SLTx platform;

    hire additional research, development and manufacturing personnel;

    continue to hire and retain clinical and commercial personnel;

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    add operational, financial, corporate development and management information systems and legal personnel, including personnel to support our product development and planned future commercialization efforts, as well as to support our transition to a public company;

    expand our facilities;

    acquire or in-license product candidates, intellectual property and technologies;

    develop an automated encapsulation system for future commercial scale manufacturing of our SLTx platform or otherwise build or expand our manufacturing capabilities or capacity, including future manufacturing facilities;

    file, prosecute, defend, and enforce our patent claims and other intellectual property rights, including patent infringement actions brought by third parties against us regarding our investigational medicines or actions by us challenging the patent or intellectual property rights of others, and provide reimbursement of third-party expenses related to our patent portfolio; and

    operate as a public company.

        We expect that it will be many years, if ever, before we have a product candidate ready for commercialization. To become and remain profitable, we must, either directly or through collaborators, develop and eventually commercialize a medicine or medicines with significant market potential. This will require us to be successful in a range of challenging activities, including identifying product candidates, completing preclinical testing and clinical trials of product candidates, obtaining marketing approval for these product candidates, manufacturing, marketing, and selling those medicines for which we may obtain marketing approval, and satisfying any post-marketing requirements. We may never succeed in these activities and, even if we do, may never generate revenues that are significant or large enough to achieve profitability. We are currently in the preclinical testing stages for all of our research programs other than SIG-001. Because of the numerous risks and uncertainties associated with developing product candidates, we are unable to predict the extent of any future losses or when we will become profitable, if at all. If we do achieve profitability, we may not be able to sustain or increase profitability on a quarterly or annual basis. Our failure to become and remain profitable would decrease the value of our company and could impair our ability to raise capital, maintain our research and development efforts, expand our business, or continue our operations. A decline in the value of our company could also cause you to lose all or part of your investment.

We will need substantial additional funding. If we are unable to raise capital when needed, we would be forced to delay, reduce, or eliminate our research and product development programs or future commercialization efforts.

        We expect our expenses to increase in connection with our ongoing activities, particularly as we identify, continue the research and development of, initiate clinical trials of, and seek marketing approval for, product candidates. We expect that our existing cash as of September 30, 2020, together with proceeds of $24.9 million from the issuance of Series B-1 convertible preferred stock in October 2020, would enable us to fund our operating expenses, capital expenditures requirements and debt service payments into the third quarter of 2021. As a result of our recurring losses and negative cash flows from operations combined with our anticipated use of cash to fund operations, we have concluded there is substantial doubt about our ability to continue as a going concern beyond the 12-month period from the issuance date of our audited financial statements for the year ended December 31, 2019. As a result, our independent registered public accounting firm included an explanatory paragraph in its report on our financial statements for the fiscal year ended December 31, 2019 with respect to this uncertainty. See "—There is substantial doubt about our ability to continue as a going concern." If we obtain marketing approval for SIG-001 or any other product candidates, we expect to incur significant commercialization expenses related to product sales, marketing, manufacturing, and distribution to the extent that such sales, marketing, manufacturing, and distribution are not the responsibility of a collaborator. Furthermore, upon the closing of this offering, we expect to incur additional costs associated with operating as a public company. Accordingly, we will need to

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obtain substantial additional funding in connection with our continuing operations. If we are unable to raise capital when needed or on attractive terms, we would be forced to delay, reduce, or eliminate our research and product development programs or future commercialization efforts.

        As of September 30, 2020, our cash was $62.6 million. We estimate that the net proceeds of this offering will be approximately $114.0 million, based on the initial public offering price of $18.00 per share, after deducting estimated underwriting discounts and commissions and offering expenses payable by us. We expect that the net proceeds from this offering, together with our existing cash, and proceeds of $24.9 million from the issuance of Series B-1 convertible preferred stock in October 2020, will enable us to fund our operating expenses and capital expenditure requirements into the fourth quarter of 2022. However, our operating plan may change as a result of factors currently unknown to us, and we may need to seek funding sooner than planned. Our future capital requirements will depend on many factors, including:

    the costs of continuing to develop our SLTx platform;

    the costs of acquiring licenses for the components of our products and engineered cell lines that will be used with our current and future product candidates;

    the scope, progress, results, and costs of discovery, preclinical development, formulation development, and clinical trials for our current and future product candidates;

    the costs of preparing, filing, and prosecuting patent applications, maintaining and enforcing our intellectual property and proprietary rights, and defending intellectual property-related claims;

    the costs, timing, and outcome of regulatory reviews associated with SIG-001 or any other product candidates;

    the costs of future activities, including product sales, medical affairs, marketing, manufacturing, distribution, coverage and reimbursement for SIG-001 or any other product candidates for which we receive regulatory approval;

    the cost of developing and expanding our manufacturing capabilities and advancing these manufacturing capabilities to manufacture product candidates that are commercially viable;

    the potential additional expenses attributable to adjusting our development plans (including any supply-related matters) due to the COVID-19 pandemic;

    our ability to establish and maintain additional collaborations on favorable terms, if at all;

    the success of any collaborations that we may establish and of our license agreements;

    the achievement of milestones or occurrence of other developments that trigger payments under any additional collaboration agreements we obtain;

    the extent to which we acquire or in-license product candidates, intellectual property and technologies; and

    the costs of operating as a public company.

        Identifying product candidates and conducting preclinical testing and clinical trials is a time-consuming, expensive, and uncertain process that takes years to complete, and we may never generate the necessary data or results required to obtain marketing approval and achieve product sales. In addition, even if we successfully identify and develop product candidates and those are approved, we may not achieve commercial success. Our commercial revenues, if any, will be derived from sales of medicines that we do not expect to be commercially available for many years, if at all. Accordingly, we will need to continue to rely on additional financing to achieve our business objectives. Adequate additional financing may not be available to us on acceptable terms, or at all.

        Any additional fundraising efforts may divert our management from their day-to-day activities, which may adversely affect our ability to develop and commercialize our product candidates. Disruptions in the

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financial markets in general and more recently due to the COVID-19 pandemic could make equity and debt financing more difficult to obtain and may have a material adverse effect on our ability to meet our fundraising needs. We cannot be certain that additional funding will be available on acceptable terms, or at all. If we are unable to raise additional capital in sufficient amounts or on terms acceptable to us, we may have to significantly delay, scale back or discontinue the development or commercialization of our product candidates or other research and development initiatives. Our license agreements and any future collaboration agreements may also be terminated if we are unable to meet the payment or other obligations under the agreements. We could be required to seek collaborators for SIG-001 and our other product candidates at an earlier stage than otherwise would be desirable or on terms that are less favorable than might otherwise be available or relinquish or license on unfavorable terms our rights to SIG-001 or any other product candidates in markets where we otherwise would seek to pursue development or commercialization ourselves.

        If we are unable to obtain funding on a timely basis, we may be required to significantly curtail, delay or discontinue one or more of our research or development programs or the commercialization of any product candidate, or be unable to expand our operations or otherwise capitalize on our business opportunities, as desired, which could materially affect our business, financial condition and results of operations. Any of the above events could significantly harm our business, prospects, financial condition and results of operations and cause the price of our common stock to decline.

Our variable rate indebtedness subjects us to interest rate risk, which could cause our debt service obligations to increase significantly.

        LIBOR and other interest rates that are indices deemed to be "benchmarks" are the subject of recent and ongoing national, international and other regulatory guidance and proposals for reform. Some of these reforms are already effective, while others are still to be implemented. These reforms may cause such benchmarks to perform differently than in the past, or to disappear entirely, or have other consequences that cannot be predicted. Any such consequence could have a material adverse effect on our existing facilities or our future debt linked to such a "benchmark" and our ability to service debt that bears interest at floating rates of interest.

Raising additional capital may cause dilution to our stockholders, including purchasers of common stock in this offering, restrict our operations or require us to relinquish rights to our technologies or product candidates we may develop.

        Until such time, if ever, as we can generate substantial product revenues, we expect to finance our cash needs through a combination of equity offerings, debt financings, collaborations, strategic alliances, and licensing arrangements. To the extent that we raise additional capital through the sale of equity or convertible debt securities, your ownership interest will be diluted, and the terms of these securities may include liquidation or other preferences that adversely affect your rights as a common stockholder. Additional debt financing, if available, may involve agreements that include covenants limiting or restricting our ability to take specific actions, such as incurring additional debt, making capital expenditures, declaring dividends, and possibly other restrictions.

        If we raise funds through additional collaborations, strategic alliances, or licensing arrangements with third parties, we may have to relinquish valuable rights to our technologies, future revenue streams, research programs, or product candidates we may develop, or we may have to grant licenses on terms that may not be favorable to us. If we are unable to raise additional funds through equity or debt financings when needed, we may be required to delay, limit, reduce, or terminate our product development or future commercialization efforts or grant rights to develop and market product candidates that we would otherwise prefer to develop and market ourselves.

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Our short operating history may make it difficult for you to evaluate the success of our business to date and to assess our future viability.

        We are an early stage company. We were founded in 2015 and commenced operations in 2016. Our operations to date have been limited to organizing and staffing our company, business planning, raising capital, acquiring and developing our SLTx platform, identifying product candidates, undertaking preclinical studies and have initiated clinical studies for SIG-001. We have not yet commenced clinical trials for any product candidate other than SIG-001, and the risk of failure for our programs is high. We have not yet demonstrated an ability to successfully complete any clinical trials, including large-scale, pivotal clinical trials, obtain marketing approvals, manufacture a commercial-scale medicine, or arrange for a third party to do so on our behalf, or conduct sales and marketing activities necessary for successful commercialization. Typically, it takes about 10 to 15 years to develop a product candidate from the time it is discovered to when it is available for treating patients, if ever. Consequently, any predictions you make about our future success or viability may not be as accurate as they could be if we had a longer operating history.

        Our limited operating history may make it difficult to evaluate our technologies and industry and predict our future performance. Our short history as an operating company makes any assessment of our future success or viability subject to significant uncertainty. We will encounter risks and difficulties frequently experienced by early stage companies in rapidly evolving fields. If we do not address these risks successfully, our business will suffer.

        In addition, we may encounter other unforeseen expenses, difficulties, complications, delays, and other known and unknown factors. We will need to transition from a company with a research focus to a company capable of supporting commercial activities. We may not be successful in such a transition.

We have never generated revenue from product sales and may never become profitable.

        Our ability to generate revenue from product sales and achieve profitability depends on our ability, alone or with collaborative partners, to successfully complete the development of, and obtain the regulatory approvals necessary to commercialize our current and future product candidates. We do not anticipate generating revenues from product sales for the next several years, if ever. Our ability to generate future revenues from product sales depends heavily on our, or our collaborators', ability to successfully:

    complete research and preclinical and clinical development of our current and future product candidates, including addressing any clinical holds that may be placed on our development activities by regulatory authorities;

    seek and obtain regulatory and marketing approvals for any of our product candidates for which we complete clinical trials;

    launch and commercialize any of our product candidates for which we obtain regulatory and marketing approval by establishing a sales force, marketing, and distribution infrastructure or, alternatively, collaborating with a commercialization partner;

    qualify for coverage and establish adequate reimbursement by government and third-party payors for any of our product candidates for which we obtain regulatory and marketing approval;

    develop, maintain, and enhance a sustainable, scalable, reproducible, and transferable manufacturing process for the product candidates we may develop;

    establish and maintain supply and manufacturing capabilities or capacities internally or with third parties that can provide adequate, in both amount and quality, products, and services to support clinical development and the market demand for any of our product candidates for which we obtain regulatory and marketing approval;

    obtain market acceptance of current or any future product candidates as viable treatment options;

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    address competing technological and market developments;

    implement internal systems and infrastructure, as needed;

    negotiate favorable terms in any collaboration, licensing, or other arrangements into which we may enter and performing our obligations in such collaborations;

    maintain, protect, enforce, defend, and expand our portfolio of intellectual property rights, including patents, trade secrets, and know-how;

    avoid and defend against third-party interference, infringement, and other intellectual property claims; and

    attract, hire, and retain qualified personnel.

        Even if one or more of our current and future product candidates are approved for commercial sale, we anticipate incurring significant costs associated with commercializing any approved product candidate. Our expenses could increase beyond expectations if we are required by the FDA or the EMA, or other regulatory authorities to perform clinical and other studies in addition to those that we currently anticipate. Even if we are able to generate revenues from the sale of any approved product candidates, we may not become profitable and may need to obtain additional funding to continue operations.

        Even if we do achieve profitability, we may not be able to sustain or increase profitability on a quarterly or annual basis. Our failure to become and remain profitable would decrease the value of our company and could impair our ability to raise capital, maintain our research and development efforts, expand our business or continue our operations. A decline in the value of our company also could cause you to lose all or part of your investment.

There is substantial doubt about our ability to continue as a going concern.

        As a result of our recurring losses and negative cash flows from operations combined with our anticipated use of cash to fund operations, we have concluded there is substantial doubt about our ability to continue as a going concern beyond the 12-month period from the issuance date of our audited financial statements for the year ended December 31, 2019. As a result, our independent registered public accounting firm included an explanatory paragraph in its report on our financial statements for the fiscal year ended December 31, 2019 with respect to this uncertainty. Our future viability as an ongoing business is dependent on our ability to generate cash from our operating activities and to raise additional capital to finance our operations. We believe that the net proceeds from this offering, together with our existing cash as of September 30, 2020, and with the proceeds of $24.9 million from the issuance of Series B-1 convertible preferred stock in October 2020, will enable us to fund our operating expenses and capital expenditures for at least the next 12 months. Without giving effect to the net proceeds from this offering, we expect that our existing cash as of September 30, 2020, together with proceeds of $24.9 million from the issuance of Series B-1 convertible preferred stock in October 2020, would enable us to fund our operating expenses, capital expenditures requirements and debt service payments into the third quarter of 2021. We have based these estimates on assumptions that may prove to be wrong, and we could exhaust our available capital resources sooner than we expect. If we are unable to raise capital when needed or on acceptable terms, we would be forced to delay, or discontinue the further development and commercialization efforts of one or more of our products, or may be forced to reduce or terminate our operations.

        There is no assurance that we will succeed in obtaining sufficient funding on terms acceptable to us to fund continuing operations, if at all. The perception that we might be unable to continue as a going concern may also make it more difficult to obtain financing for the continuation of our operations on terms that are favorable to us, or at all, and could result in the loss of confidence by investors and employees. Our financial statements do not include any adjustments that might result from the outcome of this uncertainty. If we are unable to continue as a going concern, we may have to liquidate our assets and may

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receive less than the value at which those assets are carried on our financial statements, and it is likely that our investors will lose all or a part of their investment.

Risks Related to Preclinical and Clinical Development of Our Technologies

The SLTx platform consists of novel technologies that are not yet clinically validated for human therapeutic use. The regulatory requirements applicable to our product candidates may change over time. The approaches we are taking to discover and develop novel therapeutics are unproven and may never lead to marketable products.

        The regulatory approval process for novel cellular therapy product candidates such as ours is unclear and may be lengthier and more expensive than the process for other, better-known or more extensively studied product candidates, such as biologics, small molecule drugs and other more traditional pharmaceuticals.

        Regulatory requirements governing cell therapy products have changed and may continue to change in the future. The FDA has established the Office of Tissues and Advanced Therapies, or OTAT, within its Center for Biologics Evaluation and Research, or CBER, to consolidate the review of cell therapy and related products, and has established the Cellular, Tissue and Gene Therapies Advisory Committee to advise CBER in its review. Our product candidates have been reviewed by OTAT to date, but this could change if the FDA changes any of its guidance or regulations. If we were to engage an NIH-funded institution to conduct a clinical trial, that institution's biosafety committee, a local institutional committee that reviews and oversees research utilizing recombinant or synthetic nucleic acid molecules, as well as its institutional review board, or IRB, would need to review the proposed clinical trial to assess the safety of the trial. Similarly, the EMA may issue new guidelines concerning the development and marketing authorization for cell therapy medicinal products and require that we comply with these new guidelines.

        Regulatory review committees and advisory groups, and any new guidelines they promulgate, may lengthen the regulatory review process, require us to perform additional preclinical studies or clinical trials, increase our development costs, lead to changes in regulatory positions and interpretations, delay or prevent approval and commercialization of our current or future product candidates or lead to significant post-approval limitations or restrictions. As we advance our product candidates, we will be required to consult with these regulatory and advisory groups and comply with applicable guidelines. If we fail to do so, we may be required to delay or discontinue development of our product candidates. These additional processes may result in a review and approval process that is longer than we otherwise would have expected. Delay or failure to obtain, or unexpected costs in obtaining, the regulatory approval necessary to bring a product to market could decrease our ability to generate sufficient product revenue, and our business, financial condition, results of operations and prospects would be harmed. Even if our product candidates are approved, we expect that the FDA will require us to submit follow-up data regarding our clinical trial subjects for a number of years after any approval. If this follow-up data shows negative long-term safety or efficacy outcomes for these patients, the FDA may revoke its approval or change the label of our products in a manner that could have an adverse impact on our business.

        In addition, adverse developments in clinical trials of cell therapy products conducted by others may cause the FDA or other oversight bodies to change the requirements for approval of our product candidates. As a result, it is difficult to determine how long it will take or how much it will cost to obtain regulatory approvals for our product candidates.

We may not be successful in our efforts to identify and develop product candidates. If these efforts are unsuccessful, we may never become a commercial stage company or generate any revenues.

        The success of our business depends primarily upon our ability to identify, develop, and commercialize product candidates using on our SLTx platform. We have not yet commenced clinical trials for any product candidate other than SIG-001. Because most of our programs are all in the research or preclinical stage, we have not yet been able to assess safety of our product candidates in humans, and there may be long-term effects from treatment with any of our future product candidates that we cannot predict

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at this time. Additionally, our research programs may fail to identify product candidates for clinical development for a number of reasons. Our research methodology may be unsuccessful in identifying product candidates, our product candidates may be shown to have harmful side effects in preclinical in vitro experiments or animal model studies, they may not show promising signals of therapeutic effect in such experiments or studies or they may have other characteristics that may make the product candidates impractical to manufacture, unmarketable, or unlikely to receive marketing approval.

        If any of these events occur, we may be forced to abandon our research or development efforts for a program or programs, which would have a material adverse effect on our business, financial condition, results of operations, and prospects. Research programs to identify new product candidates require substantial technical, financial, and human resources. We may focus our efforts and resources on potential programs or product candidates that ultimately prove to be unsuccessful, which would be costly and time-consuming.

We are early in our development efforts. It will be many years before we or our collaborators commercialize a product candidate, if ever. If we are unable to advance our product candidates to clinical development, obtain regulatory approval and ultimately commercialize our product candidates, or experience significant delays in doing so, our business will be materially harmed.

        We are early in our development efforts and have focused our research and development efforts to date on select indications, including rare blood disorders, lysosomal storage diseases, endocrine and other chronic disorders, when identifying our initial targeted disease indications and our initial product candidates. Lysosomal storage diseases are characterized as a set of rare inherited metabolic disorders that affect the function of the lysosome. We have initiated our first in human trial for SIG-001 and dosed our first patient in October 2020 and are conducting Investigational New Drug application, or IND-enabling studies for three of our other product candidates, but there is no guarantee that the results from such IND-enabling studies will enable us to commence clinical trials of our product candidates in a timely manner, or at all. Our future success depends heavily on the successful development of our product candidates.

        We have not submitted INDs to the FDA, or similar filings to any other regulatory agency for any product candidate other than SIG-001. We have invested substantially all of our efforts and financial resources in building our SLTx platform, and the identification and preclinical development of our current product candidates. Our ability to generate product revenue, which we do not expect will occur for many years, if ever, will depend heavily on the successful development and eventual commercialization of our product candidates, which may never occur. We currently generate no revenue from sales of any product and we may never be able to develop or commercialize a marketable product.

        In the first half of 2020, we submitted a Clinical Trial Application, or CTA, in the United Kingdom and an IND in the United States for SIG-001 for the treatment of Hemophilia A, which have been accepted by the Medicines and Healthcare products Regulatory Agency, or MHRA, and the FDA, respectively. In the event that we are required to satisfy other FDA requirements, the start of our first clinical trials in the United States may be delayed. Even after we receive and incorporate guidance from these regulatory authorities, the FDA or other regulatory authorities could disagree that we have satisfied their requirements to commence our clinical trial or change their position, which may require us to complete additional preclinical studies or clinical trials or impose stricter approval conditions than we currently expect. We have initiated enrollment and dosed our first patient for our multicenter Phase 1/2 clinical study of SIG-001 in Hemophilia A in the United Kingdom, followed by the initiation of sites and the enrollment of patients in the United States. We also plan to submit an application to initiate this study in Germany, and, if this application is cleared, to open sites for this study in Germany. In addition, we may pursue studies for SIG-001 or our other product candidates in additional jurisdictions. There are equivalent processes and risks applicable to clinical trial applications in other countries, including in Europe with respect to our CTA application for SIG-001. In addition, we may amend our current clinical trial protocol. Any subsequent protocol amendments must be submitted to the FDA or MHRA as part of the IND or CTA, respectively.

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        Commercialization of our product candidates will require additional preclinical and clinical development; regulatory and marketing approval in multiple jurisdictions, including by the FDA and the EMA; obtaining manufacturing supply, capacity and expertise; building of a commercial organization; and significant marketing efforts. The success of our current and future product candidates will depend on many factors, including the following:

    sufficiency of our financial and other resources to complete the necessary preclinical studies, IND-enabling studies, and clinical trials;

    successful completion of preclinical studies resulting in data that is supportive of advancing to an IND or CTA submission;

    successful submissions of INDs or comparable foreign applications that allow commencement of our planned or future clinical trials, including resolving any clinical holds that may be imposed on such submissions;

    successful initiation, enrollment in, and completion of, clinical trials;

    positive results from our clinical trials that support a finding of safety and effectiveness and an acceptable risk-benefit profile in the intended populations;

    receipt of marketing approvals from applicable regulatory authorities;

    establishment of arrangements with third-party manufacturers for clinical supply and commercial manufacturing and, where applicable, commercial manufacturing capabilities;

    successful development of our internal manufacturing processes and transfer to larger-scale facilities operated by either a contract manufacturing organization, or CMO, or by us;

    obtaining and maintaining patent, trade secret, and other intellectual property protection and non-patent exclusivity for our product candidates;

    launching commercial sales of the product candidates, if and when approved, whether alone or in collaboration with others;

    acceptance of the products, if and when approved, by patients, the medical community, and third-party payors;

    effectively competing with other therapies and treatment options;

    a continued acceptable safety profile of the product candidates following approval;

    enforcing and defending intellectual property and proprietary rights and claims; and

    supplying the products at a price that is acceptable to the pricing or reimbursement authorities in different countries.

        If we do not successfully achieve one or more of these activities in a timely manner or at all, we could experience significant delays or an inability to successfully commercialize any product candidates we may develop, which would materially harm our business. If we do not receive regulatory approvals for our product candidates, we may not be able to continue our operations.

We may expend our limited resources to pursue a particular product candidate or indication and fail to capitalize on product candidates or indications that may be more profitable or for which there is a greater likelihood of success.

        Because we have limited financial and managerial resources, we focus on research programs and product candidates that we identify for specific indications among many potential options. As a result, we may forego or delay pursuit of opportunities with other product candidates or for other indications that later prove to have greater commercial potential. Our resource allocation decisions may cause us to fail to

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capitalize on viable commercial products or profitable market opportunities. Our spending on current and future research and development programs and product candidates for specific indications may not yield any commercially viable medicines. If we do not accurately evaluate the commercial potential or target market for a particular product candidate, we may relinquish valuable rights to that product candidate through collaboration, licensing, or other royalty arrangements in cases in which it would have been more advantageous for us to retain sole development and commercialization rights to such product candidate. Any of the foregoing could have a material adverse effect on our business, financial condition, results of operations, and prospects.

We currently have only one clinical stage product candidate, SIG-001, for which we dosed our first patient in October 2020. A failure of this product candidate in clinical development would adversely affect our business and may require us to discontinue development of other product candidates, which are all based on the same SLTx platform.

        While we have certain preclinical programs in development, including SIG-002 for the treatment of Type 1 Diabetes, or T1D, SIG-005 for the treatment of mucopolysaccharidosis type 1, or MPS-1, and SIG-009 for the treatment of Factor VII Deficiency, or FVIID, and intend to develop other product candidates, it will take additional investment and time for such programs to reach the same stage of development as SIG-001. Since all of the product candidates in our current pipeline are based on the same SLTx platform, if SIG-001 fails in development as a result of any underlying problem with our SLTx platform, then we may be required to discontinue development of all of our product candidates. If we were required to discontinue development of SIG-001 or if SIG-001 were to fail to receive regulatory approval or were to fail to achieve sufficient market acceptance, we could be prevented from or significantly delayed in achieving profitability.

We only have preliminary data from the first patient dosed with SIG-001 and no results from our product candidates in clinical trials and any favorable preclinical results are not predictive of results that may be observed in clinical trials.

        Data obtained from preclinical and clinical activities are subject to varying interpretations and analyses, which may delay, limit or prevent regulatory approval. Many companies that have believed their product candidates performed satisfactorily in preclinical studies and clinical development have nonetheless failed to obtain marketing approval of their product candidates. As we generate preclinical results, such results will not ensure that later preclinical studies or clinical trials will demonstrate similar results.

        We have not yet initiated clinical trials for any product candidate other than SIG-001, and to date, we have not generated clinical trial results other than preliminary data from the first patient dosed with SIG-001. Our results to date have shown low single digit activity levels of FVIII. There is a high failure rate for drugs and biologics proceeding through clinical trials. Even if initial clinical trials in any of our current or future product candidates are successful, these product candidates may fail to show the desired safety and efficacy in later stages of clinical development despite having successfully advanced through preclinical studies and initial clinical trials. A number of companies in the pharmaceutical and biotechnology industries have suffered significant setbacks in later-stage clinical trials even after achieving promising results in earlier stage clinical trials.

        In addition, regulatory delays or rejections may be encountered as a result of many factors, including changes in regulatory policy during the period of product development and clinical holds that may be imposed on our clinical trials. Any such adverse events may cause us to delay, limit, or terminate planned clinical trials, any of which would have a material adverse effect on our business, financial condition, results of operations, and prospects.

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If any of the product candidates we may develop, or the delivery modes we rely on to administer them, cause serious adverse events, undesirable side effects, or unexpected characteristics, such events, side effects or characteristics could delay or prevent regulatory approval of the product candidates, limit the commercial potential, or result in significant negative consequences following any potential marketing approval.

        We are currently evaluating only one product candidate, SIG-001, in human clinical trials. Our product candidates are composed of engineered human cell lines, encapsulated in a biocompatible matrix sphere. To date, there have been no completed human clinical trials for product candidates arising from our SLTx platform or consisting of our cell or sphere technologies. There may be serious adverse events or undesirable side effects related to either component of our product candidates.

        If any product candidates we develop are associated with serious adverse events, undesirable side effects, or unexpected characteristics, we may need to abandon their development or limit development to certain uses or subpopulations in which the serious adverse events, undesirable side effects or other characteristics are less prevalent, less severe, or more acceptable from a risk-benefit perspective, any of which would have a material adverse effect on our business, financial condition, results of operations, and prospects. Many product candidates that initially showed promise in early stage testing for rare blood disorders, lysosomal storage diseases, endocrine and other chronic disorders have later been found to cause side effects that prevented further clinical development of the product candidates.

        If our clinical trials result in a high and unacceptable severity and/or prevalence of adverse events, the FDA, the EMA or other regulatory authorities could require us to cease further development of, or deny approval of, any product candidates we are able to develop for any or all targeted indications. Even if we are able to demonstrate that all future serious adverse events are not product related, such occurrences could affect patient recruitment or the ability of enrolled patients to complete the trial. Moreover, if we elect, or are required, to delay, suspend or terminate any clinical trial of any product candidate we may develop, the commercial prospects of such product candidates may be harmed and our ability to generate product revenues from any of these product candidates may be delayed or eliminated. Any of these occurrences may harm our ability to identify and develop product candidates, and may harm our business, financial condition, result of operations, and prospects significantly.

        Additionally, if we successfully develop a product candidate and it receives marketing approval, the FDA could require us to adopt a Risk Evaluation and Mitigation Strategy, or REMS, to ensure that the benefits of treatment with such product candidate outweighs the risks for each potential patient, which may include, among other things, a medication guide outlining the risks of the product for distribution to patients, a communication plan to health care practitioners, extensive patient monitoring, or distribution systems and processes that are highly controlled, restrictive, and more costly than what is typical for the industry. Furthermore, if we or others later identify undesirable side effects caused by any product candidate that we develop, several potentially significant negative consequences could result, including:

    regulatory authorities may suspend or withdraw approvals of such product candidate, or may refuse to approve supplemental applications for such product candidate;

    regulatory authorities may require additional warnings on the label, such as a "Boxed Warning" or contraindication, or limit the approved use of such product candidate;

    regulatory authorities may impose additional restriction on the marketing of, or the manufacturing processes for, the particular product candidate;

    we may be required to recall the product or change the way it is administered in patients;

    we may be required to conduct additional clinical trials;

    we could be sued and held liable for harm caused to patients; and

    our reputation may suffer.

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        Any of these events could prevent us from achieving or maintaining market acceptance of our current and future product candidates and could have a material adverse effect on our business, financial condition, results of operations, and prospectus.

If clinical trials of our current and future product candidates fail to demonstrate safety and efficacy to the satisfaction of regulatory authorities or do not otherwise produce positive results, we may incur additional costs or experience delays in completing, or ultimately be unable to complete, the development and commercialization of such product candidates.

        Before obtaining marketing approval from regulatory authorities for the sale of any of our current and future product candidates, we must complete preclinical development and then conduct extensive clinical trials to demonstrate the safety and efficacy in humans. Clinical testing is expensive, difficult to design and implement, can take many years to complete, and is uncertain as to outcome. A failure of one or more clinical trials can occur at any stage of testing. The outcome of preclinical testing and early clinical trials may not be predictive of the success of later clinical trials, and interim results of a clinical trial do not necessarily predict final results.

        We and our collaborators may experience numerous unforeseen events during, or as a result of, clinical trials that could delay or prevent our ability to complete such clinical trials, receive marketing approval or commercialize our current and future product candidates, including:

    delays in reaching a consensus with regulators on trial design;

    regulators, IRBs, or independent ethics committees may not authorize us or our investigators to commence a clinical trial or conduct a clinical trial at a prospective trial site;

    delays in reaching or failing to reach agreement on acceptable clinical trial contracts or clinical trial protocols with prospective contract research organizations, or CROs, and clinical trial sites, the terms of which can be subject to extensive negotiation and may vary significantly among different CROs and trial sites;

    difficulties in recruiting investigators of appropriate competence and experience for our clinical trials;

    the number of patients required for clinical trials of any of our current and future product candidates may be larger than we anticipate; enrollment of suitable participants in these clinical trials may be delayed or slower than we anticipate; or patients may drop out of these clinical trials at a higher rate than we anticipate;

    our third-party contractors may fail to comply with regulatory requirements or meet their contractual obligations to us in a timely manner, or at all;

    regulators, IRBs, or independent ethics committees may impose a clinical hold and require that we or our investigators suspend or terminate clinical research or clinical trials of any of our current and future product candidates for various reasons,

    noncompliance with regulatory requirements, a finding of undesirable side effects or other unexpected characteristics, or that the participants are being exposed to unacceptable health risks or after an inspection of our clinical trial operations or trial sites;

    failure to perform clinical trials in accordance with study protocols, Good Clinical Practice, or GCP, requirements, and other regulatory requirements;

    the cost of clinical trials of any of our current and future product candidates may be greater than we anticipate;

    delays in having patients complete participation in a trial or return for post-treatment follow-up;

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    delays in participation in a trial as a result of failure to deliver treatment doses to clinical trial sites in a timely manner, the logistical burden of dose delivery or failure by clinical trial sites to store treatment doses according to protocols;

    clinical trial sites deviating from trial protocol or dropping out of a trial;

    selection of clinical endpoints that require prolonged periods of clinical observation or analysis of the resulting data;

    occurrence of serious adverse events associated with any of our current and future product candidates that are viewed to outweigh their potential benefits;

    occurrence of serious adverse events in trials of the same class of agents conducted by other sponsors;

    the supply or quality of any of our current and future product candidates or other materials necessary to conduct clinical trials of any of our current and future product candidates may be insufficient or inadequate, including as a result of delays in the testing, validation, manufacturing, and delivery of any of our current and future product candidates to the clinical sites by us or by third parties with whom we have contracted to perform certain of those functions;

    changes in regulatory requirements and guidance that require amending or submitting new clinical protocols; and

    clinical trials of any of our current and future product candidates may produce negative or inconclusive results, and we may decide, or regulators may require us, to conduct additional clinical trials or abandon product development or research programs altogether.

        In addition, disruptions caused by the COVID-19 pandemic may increase the likelihood that we encounter such difficulties or delays in initiating, enrolling, conducting or completing our planned and ongoing clinical trials. We could also encounter delays if a clinical trial is suspended or terminated by us, by the IRBs of the institutions in which such trials are being conducted, by a Data Safety Monitoring Board for such trial or by the FDA or comparable foreign regulatory authorities. Such authorities may impose such a suspension or termination due to a number of factors, including failure to conduct the clinical trial in accordance with regulatory requirements or our clinical protocols, inspection of the clinical trial operations or trial site by the FDA or comparable foreign regulatory authorities resulting in the imposition of a clinical hold, unforeseen safety issues or adverse side effects, failure to demonstrate a benefit from using a drug, changes in governmental regulations or administrative actions or lack of adequate funding to continue the clinical trial. In addition, changes in regulatory requirements and policies may occur, and we may need to amend clinical trial protocols to comply with these changes. Amendments may require us to resubmit our clinical trial protocols to IRBs for reexamination, which may impact the costs, timing or successful completion of a clinical trial.

        If we or our collaborators are required to conduct additional clinical trials or other testing of any of our current and future product candidates beyond those that we currently contemplate, if we or our collaborators are unable to successfully complete clinical trials or other testing of any product candidates we may develop, or if the results of these trials or tests are not positive or are only modestly positive or if there are safety concerns, we or our collaborators may:

    be delayed in obtaining marketing approval for any such product candidates or not obtain marketing approval at all;

    lose the support of collaborators, requiring us to bear more of the costs associated with research and development;

    obtain approval for indications or patient populations that are not as broad as intended or desired;

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    obtain marketing approval in some countries but not others;

    obtain approval with labeling that includes significant use or distribution restrictions or safety warnings, including boxed warnings;

    be subject to changes in the way the product is administered;

    be required to perform additional clinical trials to support approval or be subject to additional post- marketing testing requirements;

    have regulatory authorities withdraw, or suspend, their approval of the product or impose restrictions on its distribution in the form of a REMS or through modification to an existing REMS;

    be subject to lawsuits; or

    experience damage to our reputation.

        Product development costs will also increase if we or our collaborators experience delays in clinical trials or other testing or in obtaining marketing approvals. We do not know whether our clinical trials will begin as planned, will need to be restructured, or will be completed on schedule, or at all. Significant clinical trial delays, including those caused by the COVID-19 pandemic, also could shorten any periods during which we may have the exclusive right to commercialize any product candidates we may develop, could allow our competitors to bring products to market before we do, and could impair our ability to successfully commercialize any product candidates we may develop, any of which may harm our business, financial condition, results of operations, and prospects.

        Moreover, principal investigators for our clinical trials may serve as scientific advisors or consultants to us from time to time and receive compensation in connection with such services. Under certain circumstances, we may be required to report some of these relationships to the FDA or comparable foreign regulatory authorities. The FDA or comparable foreign regulatory authority may conclude that a financial relationship between us and a principal investigator has created a conflict of interest or otherwise affected interpretation of the trial. The FDA or comparable foreign regulatory authority may therefore question the integrity of the data generated at the applicable clinical trial site and the utility of the clinical trial itself may be jeopardized. This could result in a delay in review, or rejection, of our marketing applications by the FDA or comparable foreign regulatory authority, as the case may be, and may ultimately lead to the denial of marketing approval of our product candidates.

If we experience delays or difficulties in the enrollment and dosing of patients in clinical trials, our receipt of necessary regulatory approvals could be delayed or prevented.

        Identifying and qualifying patients to participate in clinical trials of SIG-001 or any other product candidates is critical to our success. The timing of our clinical trials depends on our ability to recruit patients to participate in our studies as well as the dosing of such patients and completion of required follow-up periods. For example, hemophilia trials often take longer to enroll due to the availability of existing treatments. There are also a number of other product candidates in development by our competitors, who compete for the same limited patient populations. If we or our collaborators are unable to locate and enroll a sufficient number of eligible patients to participate in these trials as required by the FDA, the EMA or other analogous regulatory authorities outside the United States, or as needed to provide appropriate statistical power for a given trial, we may not be able to initiate or continue clinical trials for our current and future product candidates. Enrollment may be particularly challenging for some of the rare diseases we are targeting in our most advanced programs. For example, the severe or moderate Hemophilia A patient population is estimated to be approximately 10,000 patients in the United States and approximately 95,000 worldwide and the incidence of FVIID is estimated to be between one in 300,000 and one in 500,000, worldwide. The approximate incidence of MPS-1 is one in 100,000 live births and only approximately 4,000 to 5,000 patients with Fabry disease are known in the United States. Additionally, we

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may face similar challenges or delays in our other or potential future clinical trials. If patients are unwilling to participate in our studies because of negative publicity from adverse events related to the biotechnology or cell therapy, engineered cell therapy or encapsulated cell therapy fields, competitive clinical trials for similar patient populations or for other reasons, the timeline for recruiting patients, conducting studies and obtaining regulatory approval of SIG-001 or any other product candidates may be delayed. These delays could result in increased costs, delays in advancing our product candidates, delays in testing the effectiveness of our product candidates or termination of the clinical trials altogether. Furthermore, our ability to enroll patients may be significantly delayed by the evolving COVID-19 pandemic and we do not know the extent and scope of such delays at this point.

        Patient enrollment is also affected by other factors, including:

    severity of the disease under investigation;

    size of the patient population and process for identifying patients;

    design of the trial protocol;

    availability and efficacy of approved medications for the disease under investigation;

    convenience and ease of administration compared to approved medications for the disease under investigation and the willingness of patients to undergo the surgical procedures necessary to administer our product candidates, such as laparoscopy;

    ability to obtain and maintain patient informed consent;

    risk that enrolled patients will drop out before completion of the trial;

    eligibility and exclusion criteria for the trial in question;

    perceived risks and benefits of the product candidate under trial;

    efforts to facilitate timely enrollment in clinical trials;

    patient referral practices of physicians;

    ability to monitor patients adequately during and after treatment;

    proximity and availability of clinical trial sites for prospective patients; and

    factors we may not be able to control, such as current or potential pandemics that may limit patients, principal investigators or staff or clinical site availability (e.g., outbreak of COVID-19).

        Our ability to successfully initiate, enroll, and complete a clinical trial in any foreign country is subject to numerous risks unique to conducting business in foreign countries, including:

    difficulty in establishing or managing relationships with CROs and physicians;

    different standards for the conduct of clinical trials;

    different standard-of-care for patients with a particular disease;

    difficulty in locating qualified local consultants, physicians, and partners; and

    potential burden of complying with a variety of foreign laws, medical standards, and regulatory requirements, including the regulation of pharmaceutical and biotechnology products and treatment.

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        Enrollment delays in our clinical trials may result in increased development costs for SIG-001 or any other product candidates, which would cause the value of our company to decline and limit our ability to obtain additional financing. If we or our collaborators have difficulty enrolling a sufficient number of patients to conduct our clinical trials as planned, we may need to delay, limit, or terminate clinical trials for SIG-001 or our other product candidates, or expand to additional jurisdictions, which could impose additional challenges on our company and expose us to risks. If we are not successful in conducting our clinical trials as planned, it would have an adverse effect on our business, financial condition, results of operations, and prospects.

Even if we complete the necessary clinical trials, we cannot predict when, or if, we will obtain regulatory approval to commercialize SIG-001 or any other product candidate in the United States or any other jurisdiction, and any such approval may be for a more narrow indication than we seek.

        We cannot commercialize a product candidate until the appropriate regulatory authorities have reviewed and approved the product candidate. Even if SIG-001 and any other product candidates meet their safety and efficacy endpoints in clinical trials, the regulatory authorities may not complete their review processes in a timely manner, or we may not be able to obtain regulatory approval. Additional delays may result if an FDA Advisory Committee or other regulatory authority recommends non-approval or restrictions on approval. In addition, we may experience delays or rejections based upon additional government regulation from future legislation or administrative action, or changes in regulatory authority policy during the period of product development, clinical trials, and the review process.

        Regulatory authorities also may approve a product candidate for more limited indications than requested or they may impose significant limitations in the form of narrow indications, warnings or a REMS. These regulatory authorities may require labeling that includes precautions or contraindications with respect to conditions of use, or they may grant approval subject to the performance of costly post-marketing clinical trials. In addition, regulatory authorities may not approve the labeling claims that are necessary or desirable for the successful commercialization of any product candidates we may develop. Any of the foregoing scenarios could materially harm the commercial prospects for SIG-001 or any other product candidates and materially adversely affect our business, financial condition, results of operations, and prospects.

        To date, we have not submitted a biologics license application, or BLA, or other marketing authorization application to the FDA or similar drug approval submissions to comparable foreign regulatory authorities for any product candidate. Marketing approval by the FDA in the United States, if obtained, does not ensure approval by regulatory authorities in other countries or jurisdictions. In addition, clinical trials conducted in one country may not be accepted by regulatory authorities in other countries, and regulatory approval in one country does not guarantee regulatory approval in any other country. Approval processes vary among countries and can involve additional product candidate testing and validation and additional administrative review periods. Seeking foreign regulatory approval could result in difficulties and costs for us and require additional preclinical studies or clinical trials which could be costly and time-consuming. Regulatory requirements can vary widely from country to country and could delay or prevent the introduction of SIG-001 or any other product candidates in those countries. The foreign regulatory approval process involves all of the risks associated with FDA approval. We do not have any product candidates approved for sale in any jurisdiction, including international markets, and we do not have experience in obtaining regulatory approval in international markets. If we fail to comply with regulatory requirements in international markets or to obtain and maintain required approvals, or if regulatory approvals in international markets are delayed, our target market will be reduced and our ability to realize the full market potential of our product candidates will be unrealized.

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Interim, topline or preliminary data from our clinical trials that we announce or publish from time to time may change as more patient data becomes available and are subject to audit and verification procedures that could result in material changes in the final data.

        From time to time, we may publicly disclose preliminary or topline data from our preclinical studies and clinical trials, which is based on a preliminary analysis of then-available data, and the results and related findings and conclusions are subject to change following a more comprehensive review of the data related to the particular study or trial. We also make assumptions, estimations, calculations and conclusions as part of our analyses of data, and we may not have received or had the opportunity to fully and carefully evaluate all data. As a result, the topline results that we report may differ from future results of the same studies, or different conclusions or considerations may qualify such results, once additional data have been received and fully evaluated. Topline data also remain subject to audit and verification procedures that may result in the final data being materially different from the preliminary data we previously published. As a result, topline data should be viewed with caution until the final data are available.

        From time to time, we may also disclose interim data from our preclinical studies and clinical trials. Interim data from clinical trials that we may complete are subject to the risk that one or more of the clinical outcomes may materially change as patient enrollment continues and more patient data become available. To date, we only have preliminary data from the first patient dosed with SIG-001 and no results from our product candidates in clinical trials. Adverse differences between preliminary or interim data and final data could significantly harm our business prospects. Further, disclosure of interim data by us or by our competitors could result in volatility in the price of our common stock.

        Further, others, including regulatory agencies, may not accept or agree with our assumptions, estimates, calculations, conclusions, or analyses or may interpret or weigh the importance of data differently, which could impact the value of the particular program, the approvability or commercialization of the particular product candidate or product and our company in general. If the interim, topline, or preliminary data that we report differ from actual results, or if others, including regulatory authorities, disagree with the conclusions reached, our ability to obtain approval for, and commercialize, our product candidates may be harmed, which may harm our business, financial condition, results of operations, and prospects.

Our product candidates may be considered combination products involving a proprietary delivery approach, which may result in additional regulatory and other risks.

        Because our SLTx platform represents a novel approach to cell-based therapy development, we could be asked to perform additional preclinical or clinical studies, as well as develop additional manufacturing procedures and protocols, before we are able to obtain regulatory approvals for our product candidates. Our product candidates are comprised of both allogeneic human cells, which means the cells are obtained from a human donor other than the patient, and sphere components, and therefore we expect our product candidates to be regulated as biologic combination products, such as a biologic-device combination products for administration directly to the abdominal cavity or, as a novel cell-based therapies, which may subject our product candidates to additional regulatory requirements, such as CMC, preclinical or clinical requirements. If FDA regulates our product candidates as biologic-device combination products, we anticipate each component would be subject to the FDA medical requirements for that type of component. If that is the case, our delivery system device would be subject to FDA device requirements regarding design, performance, and validation, and human factor testing, as well as manufacturing requirements, including the FDA's Quality System regulations applicable to medical devices. Additionally, products that are regulated as biologic-device combination products would require coordination within the FDA for review of the product candidate's device and biologic components. The determination whether a combination product requires a single marketing application or two separate marketing applications for each component is made by the FDA on a case-by-case basis. Although a single marketing application may

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be sufficient for the approval of a combination product, the FDA may determine that separate marketing applications are necessary. This determination could significantly increase the resources and time required to bring our combination product to market. Although the FDA has systems in place for the review and approval of combination products such as ours, we may experience delays in the development and commercialization of our product candidates due to regulatory timing constraints and uncertainties in the product development and approval process, as well as coordination between two different centers within FDA responsible for review of the different components of the combination product.

Failure to obtain marketing approval in foreign jurisdictions would prevent SIG-001 or any other product candidates from being marketed in such jurisdictions, which, in turn, would materially impair our ability to generate revenue.

        In order to market and sell SIG-001 or any other product candidates in the European Union, or the EU, and other foreign jurisdictions, we or our third-party collaborators must obtain separate marketing approvals (a single one for the EU) and comply with numerous and varying regulatory requirements. The approval procedure varies among countries and can involve additional testing and/or clinical trials. The time required to obtain approval may differ substantially from that required to obtain FDA approval. The regulatory approval process outside the United States generally includes all of the risks associated with obtaining FDA approval. In addition, in many countries outside the United States, it is required that the product candidate be approved for reimbursement before the product candidate can be approved for sale in that country. We or these third parties may not obtain approvals from regulatory authorities outside the United States on a timely basis, if at all. Approval by the FDA does not ensure approval by regulatory authorities in other countries or jurisdictions, and approval by one regulatory authority outside the United States does not ensure approval by regulatory authorities in other countries or jurisdictions or by the FDA. We may not be able to file for marketing approvals and may not receive necessary approvals to commercialize our medicines in any jurisdiction, which would materially impair our ability to generate revenue.

        On June 23, 2016, the United Kingdom, or the U.K., electorate voted in favor of leaving the EU, commonly referred to as "Brexit." On January 31, 2020, the U.K.'s withdrawal from the EU became effective. However, a "Brexit transition period" was agreed between the U.K. and the EU, which will apply until December 31, 2020. During this time, the U.K. will essentially be treated as a member state of the EU, EU law will continue to apply in the U.K. and hence the EU medicinal product regulatory regime will continue to apply in the U.K. The U.K. and EU are currently negotiating a draft treaty dealing with the U.K.-EU future relationship following after January 1, 2021. The U.K. government has ruled out any extension of the Brexit transition period beyond that date. On that short timetable the U.K. and EU are likely to focus on ensuring tariff-free trade, but it is unclear whether there will be any formal bilateral regulatory alignment between the U.K. and EU rules after January 1, 2021.

        Since the regulatory framework for medicinal products in the U.K. relating to quality, safety and efficacy, clinical trials, marketing authorization, commercial sales and distribution of medicinal products is derived from EU directives and regulations, Brexit could materially impact the future regulatory regime which applies to the approval of our product candidates in the U.K. In the first instance, a separate U.K. authorization from any centralized authorization for the EU would need to be applied for. In the immediately foreseeable future, the process is likely to remain very similar to that applicable in the EU. However, given cell and gene therapy products are currently required to be authorized by the EMA under the centralized procedure as advanced therapy medicinal products, the MHRA will not have the same history of dealing with national applications for marketing authorizations for products similar to our product candidates as it does for other types of medicinal product. For existing EU centralized authorizations, it is likely that the U.K. will act unilaterally to give holders the ability to automatically convert them into a U.K. marketing authorization. This was the position of the MHRA in its now withdrawn guidance relating to the position had no transition period been agreed under the U.K.-EU

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Agreement and the U.K. had left the EU in a "no deal" Brexit. Longer term, while a significant degree of regulatory alignment is anticipated, the U.K. medicinal products regulatory regime will be likely to diverge from that in the EU.

Even if we, or any collaborators we may have, obtain marketing approvals for any product candidates we develop, the terms of approvals and ongoing regulation of our product candidates could require the substantial expenditure of resources and may limit how we, or they, manufacture and market our product candidates, which could materially impair our ability to generate revenue.

        Any product candidate for which we obtain marketing approval, along with the manufacturing processes, post-approval clinical data, labeling, packaging, distribution, import, export, adverse event reporting, storage, recordkeeping, advertising, and promotional activities for such product candidate, will be subject to extensive and ongoing requirements imposed by the FDA, EMA and other regulatory authorities. These requirements include submissions of safety and other post-marketing information and reports, facility registration and drug listing requirements, cGMP requirements relating to quality control, quality assurance and corresponding maintenance of records and documents, requirements regarding the distribution of samples to physicians and recordkeeping, and with respect to any medical device components of our product candidates, compliance with applicable provisions of the FDA's Quality System regulation. Even if marketing approval of a product candidate is granted, the approval may be subject to limitations on the indicated uses for which the product may be marketed or to the conditions of approval, or contain requirements for costly post-marketing testing and surveillance to monitor the safety or efficacy of the product.

        Accordingly, assuming we, or any collaborators we may have, receive marketing approval for one or more product candidates we develop, we, and such collaborators, and our and their contract manufacturers will continue to expend time, money, and effort in all areas of regulatory compliance, including manufacturing, production, product surveillance, and quality control. If we and such collaborators are not able to comply with post-approval regulatory requirements, we and such collaborators could have the marketing approvals for our products withdrawn by regulatory authorities and our, or such collaborators', ability to market any future products could be limited, which could adversely affect our ability to achieve or sustain profitability. Further, the cost of compliance with post-approval regulations may have a negative effect on our business, operating results, financial condition, and prospects.

        If there are changes in the application of legislation or regulatory policies, or if problems are discovered with a product or our manufacture of a product, or if we or one of our distributors, licensees or co-marketers fails to comply with regulatory requirements, the regulators could take various actions. These include issuing warning letters or untitled letters, imposing fines on us, imposing restrictions on the product or its manufacture and requiring us to recall or remove the product from the market. The regulators could also suspend or withdraw our marketing authorizations, requiring us to conduct additional clinical trials, change our product labeling or submit additional applications for marketing authorization. If any of these events occurs, our ability to sell such product may be impaired, and we may incur substantial additional expense to comply with regulatory requirements, which could materially adversely affect our business, financial condition and results of operations.

        In addition, the FDA, the EMA, and other regulatory agencies closely regulate the post-approval marketing and promotion of medicines to ensure that they are marketed only for the approved indications and in accordance with the provisions of the approved labeling. In particular, a product may not be promoted for uses that are not approved by the FDA, EMA or other regulatory agencies as reflected in the product's approved labeling. If we receive marketing approval for a product candidate, physicians may nevertheless prescribe it to their patients in a manner that is inconsistent with the approved label. If we are found to have promoted such off-label uses, we may become subject to significant liability. The FDA, the EMA and other regulatory agencies impose stringent restrictions on manufacturers' communications regarding off-label use, and if we market our medicines for off-label use, we may be subject to enforcement

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action for off-label marketing by the FDA and other federal and state enforcement agencies, including the Department of Justice. Violation of the Federal Food, Drug, and Cosmetic Act and other statutes, including the False Claims Act, and equivalent legislation in other countries relating to the promotion and advertising of prescription products may also lead to investigations or allegations of violations of federal and state and other countries' health care fraud and abuse laws and state consumer protection laws. Even if it is later determined we were not in violation of these laws, we may be faced with negative publicity, incur significant expenses defending our actions and have to divert significant management resources from other matters.

        In addition, later discovery of previously unknown problems with our products, manufacturers, or manufacturing processes, or failure to comply with regulatory requirements, may yield various negative consequences, including:

    restrictions on such products, manufacturers, or manufacturing processes;

    restrictions on the labeling or marketing of a medicine;

    restrictions on the distribution or use of a medicine;

    requirements to conduct additional post-marketing clinical trials;

    receipt of warning or untitled letters;

    withdrawal of the products from the market, or suspension of marketing approvals;

    refusal to approve pending applications or supplements to approved applications that we submit;

    recall of our approved product candidates;

    fines, restitution, or disgorgement of profits or revenue;

    restrictions on future procurements with governmental authorities;

    suspension of any ongoing clinical trials;

    refusal to permit the import or export of our medicines;

    product seizure; and

    injunctions or the imposition of civil or criminal penalties.

        Any government investigation of alleged violations of law could require us to expend significant time and resources in response and could generate negative publicity. The occurrence of any event or penalty described above may inhibit our ability to commercialize SIG-001 or any other product candidates, if approved, and adversely affect our business, financial condition, results of operations, and prospects.

        Moreover, the policies of the FDA and of other regulatory authorities may change and additional government regulations may be enacted that could prevent, limit or delay regulatory approval of our product candidates. We cannot predict the likelihood, nature or extent of government regulation that may arise from future legislation or administrative or executive action, either in the United States or abroad. If we are slow or unable to adapt to changes in existing requirements or the adoption of new requirements or policies, or if we are not able to maintain regulatory compliance, we may lose any marketing approval that we may have obtained and we may not achieve or sustain profitability.

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Disruptions at the FDA and other government agencies caused by funding shortages or global health concerns could hinder their ability to hire, retain or deploy key leadership and other personnel, or otherwise prevent new or modified products from being developed, approved or commercialized in a timely manner or at all, which could negatively impact our business.

        The ability of the FDA to review and approve new products can be affected by a variety of factors, including government budget and funding levels, statutory, regulatory, and policy changes, the FDA's ability to hire and retain key personnel and accept the payment of user fees, and other events that may otherwise affect the FDA's ability to perform routine functions. Average review times at the FDA have fluctuated in recent years as a result. In addition, government funding of other government agencies that fund research and development activities is subject to the political process, which is inherently fluid and unpredictable. Disruptions at the FDA and other agencies may also slow the time necessary for new biologics or modifications to licensed biologics to be reviewed and/or approved by necessary government agencies, which would adversely affect our business. For example, over the last several years, including for 35 days beginning on December 22, 2018, the U.S. government has shut down several times and certain regulatory agencies, such as the FDA, have had to furlough critical FDA employees and stop critical activities.

        Separately, in response to the COVID-19 pandemic, on March 10, 2020 the FDA announced its intention to postpone most inspections of foreign manufacturing facilities, and on March 18, 2020, the FDA temporarily postponed routine surveillance inspections of domestic manufacturing facilities. Subsequently, on July 10, 2020, the FDA announced its intention to resume certain on-site inspections of domestic manufacturing facilities subject to a risk-based prioritization system. The FDA intends to use this risk-based assessment system to identify the categories of regulatory activity that can occur within a given geographic area, ranging from critical inspections to resumption of all regulatory activities. Regulatory authorities outside the United States may adopt similar restrictions or other policy measures in response to the COVID-19 pandemic. If a prolonged government shutdown occurs, or if global health concerns continue to prevent the FDA or other regulatory authorities from conducting their regular inspections, reviews or other regulatory activities, it could significantly impact the ability of the FDA or other regulatory authorities to timely review and process our regulatory submissions, which could have a material adverse effect on our business.

Risks Related to Our Industry and Future Commercialization

Even if SIG-001 or any other product candidates receives marketing approval, it may fail to achieve the degree of market acceptance by physicians, patients, healthcare payors, and others in the medical community necessary for commercial success.

        The commercial success of SIG-001 or any other product candidates will depend upon the degree of market acceptance by physicians, patients, third-party payors, and others in the medical community. Even if SIG-001 or any other product candidates receives marketing approval, it may nonetheless fail to gain sufficient market acceptance by physicians, patients, healthcare payors, and others in the medical community. The degree of market acceptance of any product candidates we may develop, if approved for commercial sale, will depend on a number of factors, including:

    the efficacy and safety of such product candidates as demonstrated in clinical trials;

    the potential and perceived advantages compared to alternative treatments;

    the limitation to our targeted patient population and limitations or warnings contained in approved labeling by the FDA or other regulatory authorities;

    the ability to offer our product candidates for sale at competitive prices;

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    convenience and ease of administration compared to alternative treatments, including the logistical challenges of administering product candidates with a short shelf life and the willingness of patients to undergo the surgical procedures necessary to administer our product candidates, such as laparoscopy;

    the clinical indications for which the product candidate is approved by the FDA, the EMA, or other regulatory agencies;

    the willingness of the target patient population to try novel therapies and of physicians to prescribe these therapies;

    product labeling or product insert requirements of the FDA, the EMA, or other regulatory authorities, including any limitations or warnings contained in a product's approved labeling;

    the timing of market introduction of competitive products;

    publicity concerning our products or competing products and treatments;

    the strength of marketing and distribution support;

    sufficient third-party coverage or reimbursement; and

    the prevalence and severity of any side effects.

        Even if SIG-001 or any other product candidates are approved, such products may not achieve an adequate level of acceptance, we may not generate significant product revenues, and we may not become profitable.

If, in the future, we are unable to establish sales and marketing capabilities or enter into agreements with third parties to sell and market any product candidates we may develop, we may not be successful in commercializing those product candidates if and when they are approved.

        We do not have a sales or marketing infrastructure and have limited experience in the sale, marketing, or distribution of pharmaceutical products. To achieve commercial success for any approved medicine for which we retain sales and marketing responsibilities, we must either develop a sales and marketing organization or outsource these functions to third parties. In the future, we may choose to build a focused sales, marketing, and commercial support infrastructure to sell, or participate in sales activities with our collaborators for, some of our current and future product candidates if and when they are approved.

        There are risks involved with both establishing our own commercial capabilities and entering into arrangements with third parties to perform these services. For example, recruiting and training a sales force or reimbursement specialists is expensive and time consuming and could delay any product launch. If the commercial launch of a product candidate for which we recruit a sales force and establish marketing and other commercialization capabilities is delayed or does not occur for any reason, we would have prematurely or unnecessarily incurred these commercialization expenses. This may be costly, and our investment would be lost if we cannot retain or reposition our commercialization personnel.

        Factors that may inhibit our efforts to commercialize SIG-001 or any other product candidates on our own include:

    our inability to recruit and retain adequate numbers of effective sales, marketing, reimbursement, customer service, medical affairs, and other support personnel;

    the inability of sales personnel to obtain access to physicians to discuss our product candidates;

    the inability of reimbursement professionals to negotiate arrangements for formulary access, reimbursement, and other acceptance by payors;

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    restricted or closed distribution channels that make it difficult to distribute SIG-001 or any other product candidates to segments of the patient population;

    the lack of complementary medicines to be offered by sales personnel, which may put us at a competitive disadvantage relative to companies with more extensive product lines; and

    unforeseen costs and expenses associated with creating an independent commercialization organization.

        If we enter into arrangements with third parties to perform sales, marketing, commercial support, and distribution services, our product revenues or the profitability of these product revenues to us may be lower than if we were to market and sell any product we may develop ourselves. In addition, we may not be successful in entering into arrangements with third parties to commercialize SIG-001 or any other product candidates or may be unable to do so on terms that are favorable to us. We may have little control over such third parties, and any of them may fail to devote the necessary resources and attention to sell and market our products effectively. If we do not establish commercialization capabilities successfully, either on our own or in collaboration with third parties, we will not be successful in commercializing our products we may develop.

We face significant competition in an environment of rapid technological change, and there is a possibility that our competitors may achieve regulatory approval before us or develop therapies that are safer or more advanced or effective than ours, which may harm our financial condition and our ability to successfully market or commercialize any product candidates we may develop.

        The development and commercialization of new therapeutic biologics is highly competitive. Moreover, the engineered cell therapy field is characterized by rapidly changing technologies, significant competition, and a strong emphasis on intellectual property. We will face competition with respect to any product candidates that we may seek to develop or commercialize in the future from major pharmaceutical companies, specialty pharmaceutical companies, and biotechnology companies worldwide. Potential competitors also include academic institutions, government agencies, and other public and private research organizations that conduct research, seek patent protection, and establish collaborative arrangements for research, development, manufacturing, and commercialization.

        There are a number of large pharmaceutical and biotechnology companies that currently market and sell products or are pursuing the development of products for the treatment of the same disease indications as our product candidates, including rare blood disorders, lysosomal storage diseases, endocrine and other chronic disorders. We may face intense competition from large pharmaceutical companies with extensive resources and established relationships in these patient communities. For example, Bayer AG, or Bayer, CSL Behring, F. Hoffmann-La Roche AG, or Roche, Novo Nordisk A/S, or Novo Nordisk, Octapharma AG, or Octapharma, Pfizer Inc., or Pfizer, Sanofi S.A., or Sanofi, and Takeda Pharmaceutical Company Limited, or Takeda, among others, have developed therapies for the treatment of Hemophilia A. In addition, several large pharmaceutical companies and biotechnology companies currently market and sell products for the treatment of lysosomal storage disorders. This includes products developed by Amicus Therapeutics, Inc., BioMarin Pharmaceutical Inc., or BioMarin, and Ultragenyx Pharmaceutical Inc., or Ultragenyx, among others. Finally, the current standard of care for T1D is highly competitive and established, and includes Novo Nordisk's Levemir and Tresiba, and Sanofi's Toujeo and Lantus. There are also diabetes programs in development at ViaCyte, Inc. and Vertex Pharmaceuticals, Inc., which may compete with any therapy to treat diabetes we may develop. Any product candidates that we or our collaborators successfully develop and commercialize will compete with existing therapies and new therapies that may become available in the future that are approved to treat the same diseases for which we may obtain approval for our product candidates.

        Many of our current or potential competitors, either alone or with their collaboration partners, may have significantly greater financial resources and expertise in research and development, manufacturing, preclinical testing, conducting clinical trials, obtaining regulatory approvals, and marketing approved

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products than we do. Mergers and acquisitions in the pharmaceutical and biotechnology industries may result in even more resources being concentrated among a smaller number of our competitors. Smaller or early-stage companies may also prove to be significant competitors, particularly through collaborative arrangements with large and established companies. These competitors also compete with us in recruiting and retaining qualified scientific and management personnel and establishing clinical trial sites and patient registration for clinical trials, as well as in acquiring technologies complementary to, or necessary for, our programs. Our commercial opportunity could be reduced or eliminated if our competitors develop and commercialize product candidates that are safer, more effective, have fewer or less severe side effects, are more convenient, or are less expensive than the product candidates we may develop or that would render any of our product candidates obsolete or non-competitive. Our competitors also may obtain FDA or other regulatory approval for their product candidates more rapidly than we may obtain approval for ours, which could result in our competitors establishing a strong market position before we are able to enter the market.

        Our commercial opportunity may also be reduced or limited if we or our partners are unable to manufacture large cryopreserved lots of our products candidates in a fully automated encapsulation system efficiently. Additionally, technologies developed by our competitors may render our product candidates, or our future developments, uneconomical or obsolete, and we may not be successful in marketing SIG-001 or any other product candidates against competitors.

        In addition, we could face litigation with respect to the validity and/or scope of patents relating to our competitors' products. The availability of our competitors' products could limit the demand, and the price we are able to charge, for SIG-001 or any other product candidates. Further, intellectual property protection for human cell lines, including the engineered cell components of our product candidates are dynamic and rapidly evolving. The scope of intellectual property protection for the human cell line(s) used in our platform may be limited, and our commercial opportunity may be reduced or limited if our competitors are able to acquire or develop the same or similar cell lines.

Even if we are able to commercialize any product candidates, such products may become subject to unfavorable pricing regulations, third-party reimbursement practices, or healthcare reform initiatives, which would harm our business.

        The regulations that govern pricing, and reimbursement for new medicines vary widely from country to country. Outside the United States, some countries require approval of the sale price of a medicine before it can be marketed. In many countries, the pricing review period begins after marketing or product licensing approval is granted. In some foreign markets, prescription pharmaceutical pricing remains subject to continuing governmental control even after initial approval is granted. As a result, we might obtain marketing approval for a product candidate in a particular country, but then be subject to price regulations that delay or might even prevent our commercial launch of the product candidate, possibly for lengthy time periods, and negatively impact the revenues we are able to generate from the sale of the product candidate in that country. Adverse pricing limitations may hinder our ability to recoup our investment in one or more product candidates we may develop, even if SIG-001 or any other product candidates obtain marketing approval.

        Our ability to commercialize any product candidates successfully also will depend in part on the extent to which reimbursement for these product candidates and related treatments will be available from government authorities or healthcare program, private health plans, and other organizations. Government authorities and third-party payors, such as private health plans, decide which medications they will pay for and establish reimbursement levels. A primary trend in the U.S. healthcare industry and elsewhere is cost containment. Government authorities and third-party payors have attempted to control costs by limiting coverage and the amount of reimbursement for particular medications. Increasingly, third-party payors are challenging the prices charged for medical products and requiring that drug companies provide them with predetermined discounts from list prices. Novel medical products, if covered at all, may be subject to enhanced utilization management controls designed to ensure that the products are used only when

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medically necessary. Such utilization management controls may discourage the prescription or use of a medical product by increasing the administrative burden associated with its prescription or creating coverage uncertainties for prescribers and patients. We cannot be sure that reimbursement will be available for any product candidate that we commercialize and, if reimbursement is available, that the level of reimbursement will be adequate. Reimbursement may impact the demand for, or the price of, any product candidate for which we obtain marketing approval. If reimbursement is not available or is available only to limited levels, we may not be able to successfully commercialize any product candidate for which we obtain marketing approval.

        There may be significant delays in obtaining reimbursement for newly approved product candidates, and coverage may be more limited than the purposes for which the product candidate is approved by the FDA, the EMA or other regulatory authorities outside the United States. Moreover, eligibility for reimbursement does not imply that any product candidate will be paid for in all cases or at a rate that covers our costs, including research, development, manufacture, sale, and distribution. Interim reimbursement levels for new product candidates, if applicable, may also not be sufficient to cover our costs and may not be made permanent. Reimbursement rates may vary according to the use of the product candidate and the clinical setting in which it is used, may be based on reimbursement levels already set for lower cost therapies or medicines and may be incorporated into existing payments for other services. Net prices for product candidates may be reduced by mandatory discounts or rebates required by government healthcare programs or private payors and by any future relaxation of laws that presently restrict imports of medicines from countries where they may be sold at lower prices than in the United States. Our inability to promptly obtain coverage and profitable payment rates from both government-funded and private payors for any approved product candidates we may develop could have a material adverse effect on our operating results, our ability to raise capital needed to commercialize medicines, and our overall financial condition.

Any product candidates for which we intend to seek approval as biologic products may face competition sooner than anticipated.

        The Patient Protection and Affordable Care Act, signed into law on March 23, 2010, includes a subtitle called the Biologics Price Competition and Innovation Act of 2009, or BPCIA, which created an abbreviated approval pathway for biological products that are biosimilar to or interchangeable with an FDA-licensed reference biological product. Under the BPCIA, an application for a biosimilar product may not be submitted to the FDA until four years following the date that the reference product was first licensed by the FDA. In addition, the approval of a biosimilar product may not be made effective by the FDA until 12 years from the date on which the reference product was first licensed. During this 12-year period of exclusivity, another company may still market a competing version of the reference product if the FDA approves a full BLA for the competing product containing the sponsor's own preclinical data and data from adequate and well-controlled clinical trials to demonstrate the safety, purity and potency of their product. The law is complex and is still being interpreted and implemented by the FDA. As a result, its ultimate impact, implementation, and meaning are subject to uncertainty. While it is uncertain when such processes intended to implement BPCIA may be fully adopted by the FDA, any such processes could have a material adverse effect on the future commercial prospects for our biological products.

        There is a risk that any of our product candidates approved as a biological product under a BLA would not qualify for the 12-year period of exclusivity or that this exclusivity could be shortened due to congressional action or otherwise, or that the FDA will not consider our product candidates to be reference products for competing products, potentially creating the opportunity for generic competition sooner than anticipated. Other aspects of the BPCIA, some of which may impact the BPCIA exclusivity provisions, have also been the subject of recent litigation. Moreover, the extent to which a biosimilar, once approved, will be substituted for any one of our reference products in a way that is similar to traditional generic substitution for non-biological products is not yet clear, and will depend on a number of marketplace and regulatory factors that are still developing.

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Due to the novel nature of our technologies and the potential for SIG-001 or any other product candidates to offer therapeutic benefit in a single administration or limited number of administrations, we face uncertainty related to pricing and reimbursement for these product candidates.

        If we are unable to obtain adequate levels of reimbursement, our ability to successfully market and sell any such product candidates will be adversely affected. The manner and level at which reimbursement is provided for services related to SIG-001 or any other product candidates (e.g., for administration of our product candidate to patients) is also important. Inadequate reimbursement for such services may lead to physician and payor resistance and adversely affect our ability to market or sell SIG-001 or any other product candidates. In addition, we may need to develop new reimbursement models in order to realize adequate value.

        Payors may not be able or willing to adopt such new models, and patients may be unable to afford that portion of the cost that such models may require them to bear. If we determine such new models are necessary but we are unsuccessful in developing them, or if such models are not adopted by payors, our business, financial condition, results of operations, and prospects could be adversely affected.

        We expect that coverage and reimbursement by government and private payors will be essential for most patients to be able to afford these treatments. Accordingly, sales of any such product candidates will depend substantially, both domestically and abroad, on the extent to which the costs of SIG-001 or any other product candidates will be paid by government authorities, private health plans, and other third-party payors. Payors may not be willing to pay high prices for a single administration. Coverage and reimbursement by a third-party payor may depend upon several factors, including the third-party payor's determination that use of a product is:

    a covered benefit under its health plan;

    safe, effective, and medically necessary;

    appropriate for the specific patient;

    cost-effective; and

    neither experimental nor investigational.

        Obtaining coverage and reimbursement for a product from third-party payors is a time-consuming and costly process that could require us to provide to the payor supporting scientific, clinical, and cost-effectiveness data. There is significant uncertainty related to third-party coverage and reimbursement of newly approved products. We may not be able to provide data sufficient to gain acceptance with respect to coverage and reimbursement. If coverage and reimbursement are not available, or are available only at limited levels, we may not be able to successfully commercialize any product candidates we may develop. Even if coverage is provided, the approved reimbursement amount may not be adequate to realize a sufficient return on our investment.

        Moreover, the downward pressure on healthcare costs in general, particularly prescription drugs and surgical procedures and other treatments, has become intense. As a result, increasingly high barriers are being erected to the entry of new product candidates such as ours. If we are unable to obtain adequate levels of reimbursement, our ability to successfully market and sell SIG-001 or any other product candidates will be harmed.

If the market opportunities for SIG-001 or any other product candidates are smaller than we believe they are, our potential revenues may be adversely affected, and our business may suffer.

        We focus certain research and product development pipelines and our product candidates on treatments for rare diseases including rare blood disorders, lysosomal storage diseases, endocrine and other chronic disorders. Our understanding of both the number of people who have these diseases, as well as the subset of people with these diseases who have the potential to benefit from treatment with our

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product candidates, are based on estimates. For example, the severe or moderate Hemophilia A patient population is estimated to be approximately 10,000 patients in the United States and approximately 95,000 worldwide and the incidence of FVIID is estimated to be between one in 300,000 and one in 500,000, worldwide. The approximate incidence of MPS-1 is one in 100,000 live births and only approximately 4,000 to 5,000 patients with Fabry disease are known in the United States. These estimates may prove to be incorrect and new studies may reduce the estimated incidence or prevalence of these diseases. The number of patients in the United States and elsewhere may turn out to be lower than expected, may not be otherwise amenable to treatment with SIG-001 or our other product candidates or patients may become increasingly difficult to identify and access, all of which would adversely affect our business, financial condition, results of operations and prospects.

Product liability lawsuits against us could cause us to incur substantial liabilities and could limit commercialization of any medicines that we may develop.

        We face an inherent risk of product liability exposure related to the testing in human clinical trials of SIG-001 or any other product candidates and will face an even greater risk if we commercially sell any product candidates that we may develop. If we cannot successfully defend ourselves against claims that our product candidates caused injuries, we could incur substantial liabilities. Regardless of merit or eventual outcome, liability claims may result in:

    decreased demand for any product candidates that we may develop;

    injury to our reputation and significant negative media attention;

    withdrawal of clinical trial participants;

    significant time and costs to defend the related litigation;

    substantial monetary awards to trial participants or patients;

    loss of revenue; and

    the inability to commercialize any product candidates that we may develop.

        Although we maintain product liability insurance coverage, it may not be adequate to cover all liabilities that we may incur. We anticipate that we will need to increase our insurance coverage when we begin clinical trials and if we successfully commercialize any medicine. Insurance coverage is increasingly expensive. We may not be able to maintain insurance coverage at a reasonable cost or in an amount adequate to satisfy any liability that may arise.

If we or any contract manufacturers and suppliers we engage fail to comply with environmental, health, and safety laws and regulations, we could become subject to fines or penalties or incur costs that could have a material adverse effect on the success of our business.

        We and any contract manufacturers and suppliers we engage are subject to numerous federal, state, and local environmental, health, and safety laws, regulations, and permitting requirements, including those governing laboratory procedures; the generation, handling, use, storage, treatment, and disposal of hazardous and regulated materials and wastes; the emission and discharge of hazardous materials into the ground, air, and water; and employee health and safety. Our operations involve the use of hazardous and flammable materials, including chemicals and biological materials. Our operations also produce hazardous waste. We generally contract with third parties for the disposal of these materials and wastes. We cannot eliminate the risk of contamination or injury from these materials. In the event of contamination or injury resulting from our use of hazardous materials, we could be held liable for any resulting damages, and any liability could exceed our resources. Under certain environmental laws, we could be held responsible for costs relating to any contamination at our current or past facilities and at third-party facilities. We also could incur significant costs associated with civil or criminal fines and penalties.

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        Compliance with applicable environmental laws and regulations may be expensive, and current or future environmental laws and regulations may impair our research and product development efforts. In addition, we cannot entirely eliminate the risk of accidental injury or contamination from these materials or wastes. Although we maintain workers' compensation insurance to cover us for costs and expenses we may incur due to injuries to our employees resulting from the use of hazardous materials, this insurance may not provide adequate coverage against potential liabilities. We do not carry specific biological or hazardous waste insurance coverage, and our property, casualty, and general liability insurance policies (under which we currently have an aggregate of approximately $15.0 million in coverage) specifically exclude coverage for damages and fines arising from biological or hazardous waste exposure or contamination. Accordingly, in the event of contamination or injury, we could be held liable for damages or be penalized with fines in an amount exceeding our resources, and our clinical trials or regulatory approvals could be suspended, which could have a material adverse effect on our business, financial condition, results of operations, and prospects.

        In addition, we may incur substantial costs in order to comply with current or future environmental, health, and safety laws, regulations, and permitting requirements. These current or future laws, regulations, and permitting requirements may impair our research, development, or production efforts. Failure to comply with these laws, regulations, and permitting requirements also may result in substantial fines, penalties, or other sanctions or business disruption, which could have a material adverse effect on our business, financial condition, results of operations, and prospects.

        Any third-party contract manufacturers and suppliers we engage will also be subject to these and other environmental, health, and safety laws and regulations. Liabilities they incur pursuant to these laws and regulations could result in significant costs or an interruption in operations, which could have a material adverse effect on our business, financial condition, results of operations, and prospects.

Our technologies are novel, and any product candidates we develop may be complex and difficult to manufacture on a clinical or commercial scale. We could experience delays in satisfying regulatory authorities or production problems that result in delays in our development or commercialization programs, limit the supply of our product candidates we may develop, or otherwise harm our business.

        Our SLTx platform is novel and the manufacture of products on the basis of our platform is untested at a large scale. Any current and future product candidates will likely require processing steps that are more complex than those required for most chemical pharmaceuticals and traditional biologics. Moreover, unlike small molecules, the physical and chemical properties of various components in our product candidates generally cannot be fully characterized. As a result, assays of the finished product candidate may not be sufficient to ensure that the product candidate will perform in the intended manner. Problems with the manufacturing process, even minor deviations from the normal process, could result in product defects or manufacturing failures that result in lot failures, product recalls, product liability claims, insufficient inventory, or potentially delay progression of our regulatory filings. If we successfully develop product candidates, we may encounter problems achieving adequate quantities and quality of clinical-grade materials that meet FDA, EMA or other comparable applicable foreign standards or specifications with consistent and acceptable production yields and costs. If we or our contract manufacturers are unable to scale our manufacturing at the same levels of quality and efficiency, we may not be able to supply the required number of doses for clinical trials or commercial supply, and our business could be harmed.

        As product candidates proceed through preclinical studies to clinical trials towards potential approval and commercialization, it is common that various aspects of the development program, such as manufacturing methods and formulation, are tested and then altered along the way in an effort to optimize processes and results. We may make changes to our manufacturing methods as part of our product development activities. Any such changes could cause any product candidates we may develop to perform differently and affect the results of clinical trials conducted with the materials manufactured using altered processes. Such changes may also require additional testing, FDA notification or FDA approval. This could delay completion of clinical trials, require the conduct of bridging clinical trials or the repetition of

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one or more clinical trials, increase clinical trial costs, delay approval of our product candidates and jeopardize our ability to commence sales and generate revenue.

        In addition, the FDA, the EMA, and other regulatory authorities may require us to submit samples of any lot of any approved product together with the protocols showing the results of applicable tests at any time. Under some circumstances, the FDA, the EMA, or other regulatory authorities may require that we not distribute a lot until the agency authorizes its release. Slight deviations in the manufacturing process, including those affecting quality attributes and stability of encapsulation, may result in unacceptable changes in the product that could result in lot failures or product recalls. Lot failures or product recalls could cause us to delay clinical trials or product launches, which could be costly to us and otherwise harm our business, financial condition, results of operations, and prospects.

        We also may encounter problems hiring and retaining the experienced scientific, quality control, and manufacturing personnel needed to manage our manufacturing process, which could result in delays in our production or difficulties in maintaining compliance with applicable regulatory requirements.

        The manufacture of biopharmaceutical products is complex and requires significant expertise, including the development of advanced manufacturing techniques and process controls. Manufacturers of cell therapy products often encounter difficulties in production, particularly in scaling out and validating initial production and ensuring the absence of contamination. Given the nature of biologics manufacturing and the cell therapy products used in our early stage programs, such as the Phase 1/2 clinical trial of SIG-001 that incorporates fresh, rather than cryopreserved, cells, there is a risk of contamination during manufacturing. For example, given the aseptic controls required for the manufacture of our product candidates, if contaminants are discovered in our supply of product candidates or in the manufacturing facilities, such manufacturing facilities may need to be closed for an extended period of time to investigate and remedy the contamination. Any such contamination could materially harm our ability to produce product candidates on schedule and could delay our development programs and results of operations and cause reputational damage. We cannot assure you that any such issues relating to the manufacture of SIG-001 or any other product candidate will not occur in the future or that significant delays would not occur as a result of any such issue.

        In addition, some of the raw materials that we anticipate will be required in our manufacturing process are derived from biologic sources. For example, engineered human cell lines serve as components of product candidates developed using the SLTx platform, and our alginates, which are naturally occurring polymers derived from seaweed. Such raw materials can be difficult to procure and may be subject to contamination or recall. A material shortage, recall, or restriction on the use of biologically derived substances in the manufacture of SIG-001 or any other product candidates could adversely impact or disrupt the commercial manufacturing or the production of clinical material, which could materially harm our development timelines and our business, financial condition, results of operations, and prospects.

        Any problems in our manufacturing process or the facilities with which we contract to make, store or ship our product candidates or any problems caused by us, our vendors or other factors not in our control could result in the loss of usable product or prevent or delay the delivery of product candidates to patients in our clinical trials, including the Phase 1/2 clinical trial of SIG-001. Any such loss or delay could materially delay our development timelines and harm our business, financial condition and results of operations. Such losses or delays could also make us a less attractive collaborator for potential partners, including larger pharmaceutical companies and academic research institutions, which could limit our access to additional attractive development programs. Problems in third-party manufacturing process or facilities also could restrict our ability to ensure sufficient clinical material for any clinical trials we may be conducting or are planning to conduct and meet market demand for any product candidates we develop and commercialize.

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We purchase some of the starting material for our product candidates from a single source or a limited number of suppliers, and the partial or complete loss of one of these suppliers could cause production delays, clinical trial delays, substantial loss of revenue and contract liability to third parties.

        We source a critical raw material used in our sphere alginate from a single supplier. A limited supply of this raw material could cause production delays, clinical trial delays, substantial lost revenue opportunities or contract liabilities to third parties. For example, there are only a limited number of qualified suppliers, and in some cases single source suppliers, for the raw materials included in our SLTx platform, including our current supply of alginates. Any interruption in supply, diminution in quality of raw materials supplied to us or failure to procure such raw materials on commercially feasible terms, including as a result of the COVID-19 pandemic, could harm our business by delaying our clinical trials, impeding commercialization of potential approved products or increasing our costs.

        Additionally, our sphere alginate is derived from a naturally occurring seaweed. The availability or characteristics of this material may be impacted by disease to this species of seaweed, ocean pollution and climate change as a result of global warming.

Risks Related to Our Relationships with Third Parties

We expect to rely on third parties to conduct our clinical trials and conduct some aspects of our research and preclinical testing, and those third parties may not perform satisfactorily, including failing to meet deadlines for the completion of such trials, research, or testing.

        We expect to rely on third parties, such as CROs, medical institutions, and clinical investigators, to conduct our clinical trials. We currently rely and expect to continue to rely on third parties to conduct some aspects of our research and preclinical testing. Any of our collaborators and partners may terminate their engagements with us at any time. If we need to enter into alternative arrangements, it may delay our product development activities.

        Our reliance on third parties for research and development activities will reduce our control over these activities but will not relieve us of our responsibilities. For example, we will remain responsible for ensuring that each of our clinical trials is conducted in accordance with study protocol for the trial. Moreover, the FDA, EMA and other regulatory authorities require us to comply GCP requirements for conducting, recording, and reporting the results of clinical trials to assure that data and reported results are credible and accurate and that the rights, integrity, and confidentiality of trial participants are protected. Regulatory authorities enforce these GCPs through periodic inspections of trial sponsors, principal investigators and trial sites. If we or any of our CROs fail to comply with applicable GCP, the clinical data generated in our clinical trials may be deemed unreliable and the FDA or comparable foreign regulatory authorities may require us to perform additional clinical trials before approving our marketing applications. We cannot assure you that upon inspection by a given regulatory authority, such regulatory authority will determine that any of our clinical trials comply with GCP regulations. In addition, our clinical trials must be conducted with product produced under cGMP regulations. Our failure to comply with these regulations may require us to repeat clinical trials, which would delay the regulatory approval process.

        We also are required to register ongoing clinical trials and post the results of completed clinical trials on government-sponsored databases, including ClinicalTrials.gov in the United States and the EudraCT database in the EU, within certain timeframes. Failure to do so can result in fines, adverse publicity, and civil and criminal sanctions.

        Although we intend to design the clinical trials for the majority of our product candidates, CROs will conduct some or all of the clinical trials. As a result, many important aspects of our development programs, including their conduct and timing, will be outside of our direct control. Our reliance on third parties to conduct future preclinical studies and clinical trials will also result in less direct control over the management of data developed through preclinical studies and clinical trials than would be the case if we were relying entirely upon our own staff.

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        Communicating with outside parties can also be challenging, potentially leading to mistakes as well as difficulties in coordinating activities. Outside parties may:

    have staffing difficulties;

    fail to comply with contractual obligations;

    experience regulatory compliance issues;

    undergo changes in priorities or become financially distressed; or

    form relationships with other entities, some of which may be our competitors.

        These factors may materially adversely affect the willingness or ability of third parties to conduct our preclinical studies and clinical trials and may subject us to unexpected cost increases that are beyond our control. If the CROs and other third parties do not perform preclinical studies and future clinical trials in a satisfactory manner, breach their obligations to us or fail to comply with regulatory requirements, the development, regulatory approval and commercialization of our product candidates may be delayed or prevented, we may not be able to obtain regulatory approval and commercialize our product candidates, or our development programs may be materially and irreversibly harmed.

        We also expect to rely on other third parties to store and distribute product supplies for our clinical trials. Any performance failure on the part of our distributors could delay clinical development or marketing approval of SIG-001 or any other product candidates or commercialization of our medicines, producing additional losses and depriving us of product revenue.

        If any of our relationships with these third-party CROs terminate, we may not be able to enter into arrangements with alternative CROs or to do so on commercially reasonable terms. If CROs do not successfully carry out their contractual duties or obligations or meet expected deadlines, if they need to be replaced or if the quality or accuracy of the clinical data they obtain is compromised due to the failure to adhere to our clinical protocols, regulatory requirements or for other reasons, our clinical trials may be extended, delayed or terminated and we may not be able to obtain regulatory approval for or successfully commercialize our product candidates. As a result, our results of operations and the commercial prospects for our product candidates would be harmed, our costs could increase and our ability to generate revenues could be delayed.

        Switching or adding additional CROs involves additional cost and requires management time and focus. In addition, there is a natural transition period when a new CRO commences work. As a result, delays occur, which can materially impact our ability to meet our desired clinical development timelines. Additionally, CROs may lack the capacity to absorb higher workloads or take on additional capacity to support our needs. Though we carefully manage our relationships with our CROs, there can be no assurance that we will not encounter similar challenges or delays in the future or that these delays or challenges will not have a material adverse impact on our business, financial condition and prospects.

Because we rely on third-party manufacturing and supply partners, our supply of research and development, preclinical and clinical development materials may become limited or interrupted or may not be of satisfactory quantity or quality.

        We rely on third-party contract manufacturers, or CMOs, to manufacture our preclinical product candidate supplies and will rely on CMOs to manufacture our clinical trial supplies. We lack the internal resources and the capability to manufacture any product candidates on a clinical or commercial scale. The facilities used by our CMOs to manufacture our product candidates must be acceptable to the FDA and other comparable foreign regulatory agencies pursuant to inspections that would be conducted after we submit our marketing application or relevant foreign regulatory submission to the applicable regulatory agency. There can be no assurance that our preclinical and clinical development product supplies will not be limited, interrupted or of satisfactory quality or continue to be available at acceptable prices. If our

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CMOs cannot successfully manufacture material that conforms to our specifications and the strict regulatory requirements of the FDA or applicable foreign regulatory agencies, they will not be able to secure or maintain regulatory approval for their manufacturing facilities. Any replacement of our CMOs could require significant effort and expertise because there may be a limited number of qualified replacements.

        The manufacturing process for our product candidates is subject to FDA and foreign regulatory authority review. Suppliers and manufacturers must meet applicable manufacturing requirements and undergo rigorous facility and process validation tests required by regulatory authorities in order to comply with regulatory standards, such as cGMP. We have no direct control over our CMOs' ability to maintain adequate quality control, quality assurance and qualified personnel. In the event that any of our manufacturers fails to comply with regulatory requirements or to perform its obligations to us in relation to quality, timing or otherwise, or if our supply of components or other materials becomes limited or interrupted for other reasons, we may be forced to manufacture the materials ourselves, for which we currently do not have the capabilities or resources, or enter into an agreement with another third party, which we may not be able to do on reasonable terms, if at all. In some cases, the technical skills or technology required to manufacture our product candidates may be unique or proprietary to the original manufacturer and we may have difficulty transferring such skills or technology to another third party and a feasible alternative may not exist. These factors would increase our reliance on such manufacturer or require us to obtain a license from such manufacturer in order to have another third party manufacture our product candidates. If we are required to change manufacturers for any reason, we will be required to verify that the new manufacturer maintains facilities and procedures that comply with quality standards and with all applicable regulations and guidelines. The delays associated with the verification of a new manufacturer could negatively affect our ability to develop product candidates in a timely manner or within budget. Our reliance on contract manufacturers also exposes us to the possibility that they, or third parties with access to their facilities, will have access to and may appropriate our trade secrets or other proprietary information.

        We expect to continue to rely on third-party CMOs if we receive regulatory approval for any product candidate. To the extent that we have existing, or enter into future, manufacturing arrangements with third parties, we will depend on these third parties to perform their obligations in a timely manner consistent with contractual and regulatory requirements, including those related to quality control and assurance. If we are unable to obtain or maintain third-party manufacturing for our product candidates, or to do so on commercially reasonable terms, we may not be able to develop and commercialize our product candidates successfully. Our or a third party's failure to execute on our manufacturing requirements and comply with cGMP could adversely affect our business in a number of ways, including:

    an inability to initiate or continue clinical studies of product candidates under development;

    delay in submitting regulatory applications, or receiving regulatory approvals, for our product candidates;

    loss of the cooperation of an existing or future strategic partner;

    subjecting third-party manufacturing facilities or our manufacturing facilities to additional inspections by regulatory authorities;

    a requirement to cease distribution or to recall batches of our product candidates; and

    in the event of approval to market and commercialize a product candidate, an inability to meet commercial demands for our products.

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We, or our third-party manufacturers, may be unable to successfully scale-up manufacturing of our product candidates in sufficient quality and quantity, which would delay or prevent us from developing our product candidates and commercializing approved products, if any.

        In order to conduct clinical studies of our product candidates and commercialize any approved product candidates, we, or our manufacturing partners, will need to manufacture them in large quantities. We, or our manufacturing partners, may be unable to successfully increase the manufacturing capacity for any of our product candidates in a timely or cost-effective manner, or at all. In addition, quality issues may arise during scale-up activities. If we, or any manufacturing partners, are unable to successfully scale up the manufacture of our product candidates in sufficient quality and quantity, the development, testing and clinical studies of that product candidate may be delayed or infeasible, and regulatory approval or commercial launch of any resulting product may be delayed or not obtained, which could significantly harm our business. We are currently evaluating which third-party manufactures to engage for scale-up to commercial supply of our product candidates, including SIG-001. If we are unable to obtain or maintain third-party manufacturing for commercial supply of product candidates, or to do so on commercially reasonable terms, or if we are unable to develop our own manufacturing capabilities, we may not be able to develop and commercialize our product candidates successfully.

We have entered and may in the future enter into collaborations with third parties for the research, development, and commercialization of SIG-002 or any other potential product candidates. If any such collaborations are not successful or our existing partners do not perform as expected, we may not be able to capitalize on the market potential of those product candidates.

        We have engaged and may in the future seek third-party collaborators for the research, development, and commercialization of certain of the product candidates we may develop. For example, pursuant to our agreement with Eli Lilly and Company, or Lilly, for the development of SIG-002, Lilly will be responsible for submitting an IND and all clinical development and commercialization activities following such IND submission. We will therefore depend on Lilly to design and conduct their clinical studies. If we enter into similar collaboration agreements for any of our other product candidates, we may also depend on partners to design and conduct clinical trials. As a result, we may have limited control over the amount and timing of resources that our collaborators dedicate to the development or commercialization of SIG-002 or other product candidates we may decide to partner with third-party collaborators. Our ability to generate revenues from these arrangements will depend on our collaborators' abilities to successfully perform the functions assigned to them in these arrangements. We cannot predict the success of our current or any future collaboration that we enter into.

        Our current and any future collaborations involving our research programs or our current or any future product candidates pose numerous risks to us, including the following:

    Collaborators may have significant discretion in determining the efforts and resources that they will apply to these collaborations.

    Collaborators may not pursue development and commercialization of any current or future product candidates or may elect not to continue or renew development or commercialization programs based on clinical trial results, changes in the collaborator's strategic focus or available funding or external factors such as an acquisition that diverts resources or creates competing priorities.

    Collaborators may delay clinical trials, provide insufficient funding for a clinical trial program, stop a clinical trial or abandon a product candidate, repeat or conduct new clinical trials, or require a new formulation of a product candidate for clinical testing.

    Collaborators could independently develop, or develop with third parties, products that compete directly or indirectly with any current or future product candidates if the collaborators believe that

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      competitive products are more likely to be successfully developed or can be commercialized under terms that are more economically attractive than ours.

    Collaborators with marketing and distribution rights to one or more product candidates may not commit sufficient resources to the marketing and distribution of such product candidates.

    Collaborators may not properly obtain, maintain, enforce, or defend our intellectual property or proprietary rights or may use our proprietary information in such a way as to invite litigation that could jeopardize or invalidate our proprietary information or expose us to potential litigation.

    Disputes may arise between the collaborators and us that result in the delay or termination of the research, development, or commercialization of our product candidates or that result in costly litigation or arbitration that diverts management attention and resources.

    We may lose certain valuable rights under circumstances identified in our collaborations, including if we undergo a change of control.

    Collaborations may be terminated and, if terminated, may result in a need for additional capital to pursue further development or commercialization of the applicable product candidates we may develop.

    Collaboration agreements may not lead to development or commercialization of product candidates in the most efficient manner or at all. If a present or future collaborator of ours were to be involved in a business combination, the continued pursuit and emphasis on our product development or commercialization program under such collaboration could be delayed, diminished, or terminated.

        If our collaborations do not result in the successful development and commercialization of product candidates, or if Lilly or any of our other collaborators terminates its agreement with us, we may not receive any future research funding or milestones or royalty payments under the collaboration. If we do not receive the funding we expect under these agreements, our development of product candidates could be delayed, and we may need additional resources to develop product candidates. In addition, if one of our collaborators terminates its agreement with us, we may find it more difficult to find a suitable replacement collaborator or attract new collaborators, and our development programs may be delayed or the perception of us in the business and financial communities could be adversely affected. All of the risks relating to product development, regulatory approval, and commercialization described in this prospectus apply to the activities of our collaborators.

        These relationships, or those like them, may require us to incur non-recurring and other charges, increase our near- and long-term expenditures, issue securities that dilute our existing stockholders, or disrupt our management and business. In addition, we could face significant competition in seeking appropriate collaborators, and the negotiation process is time-consuming and complex. Our ability to reach a definitive collaboration agreement will depend, among other things, upon our assessment of the collaborator's resources and expertise, the terms and conditions of the proposed collaboration, and the proposed collaborator's evaluation of several factors. If we license rights to any of our future product candidates, as we have with Lilly for the development and commercialization of SIG-002, we may not be able to realize the benefit of such transactions if we are unable to successfully integrate them with our existing operations and company culture.

        If conflicts arise between our partners and us, the other party may act in a manner adverse to us and could limit our ability to implement our strategies. If any of our partners terminate or breach our agreements with them, or otherwise fail to complete their obligations in a timely manner, it may have a detrimental effect on our financial position by reducing or eliminating the potential for us to receive technology access and license fees, milestones and royalties, reimbursement of development costs, as well as possibly requiring us to devote additional efforts and incur costs associated with pursuing internal development of product candidates. Furthermore, if our partners do not prioritize and commit sufficient

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resources to programs associated with our product candidates or collaboration product candidates, we or our partners may be unable to commercialize these product candidates, which would limit our ability to generate revenue and become profitable. We have issued Lilly shares of our preferred stock in connection with our collaboration. See "—Insiders will continue to have substantial influence over us after this offering, which could limit your ability to affect the outcome of key transactions, including a change of control."

If we are not able to establish collaborations on commercially reasonable terms, we may have to alter our development and commercialization plans.

        Our product development and research programs and the potential commercialization of any of our current and future product candidates will require substantial additional cash to fund expenses. For some of the product candidates we may develop, we may decide to collaborate with other pharmaceutical and biotechnology companies for the development and potential commercialization of those product candidates. For example, we have partnered with Lilly for the development and commercialization of SIG-002 for the treatment of T1D.

        We face significant competition in seeking appropriate collaborators. Whether we reach a definitive agreement for a collaboration will depend, among other things, upon our assessment of the collaborator's resources and expertise, the terms and conditions of the proposed collaboration, and the proposed collaborator's evaluation of a number of factors. Those factors may include preclinical results, the design or results of clinical trials, the likelihood of approval by the FDA, the EMA or similar regulatory authorities outside the United States, the potential market for the subject product candidate, the costs and complexities of manufacturing and delivering such product candidate to patients, the potential of competing products, the existence of uncertainty with respect to our ownership of technology, which can exist if there is a challenge to such ownership without regard to the merits of the challenge, and industry and market conditions generally. The collaborator may also consider alternative product candidates or technologies for similar indications that may be available to collaborate on and whether such a collaboration could be more attractive than the one with us.

        Collaboration agreements may also restrict us from entering into future agreements on certain terms with potential collaborators. Collaborations are complex and time-consuming to negotiate and document. In addition, there have been a significant number of recent business combinations among large pharmaceutical companies that have resulted in a reduced number of potential future collaborators.

        We may not be able to negotiate collaborations on a timely basis, on acceptable terms, or at all. If we are unable to do so, we may have to curtail the development of the product candidate for which we are seeking to collaborate, reduce or delay its development program or one or more of our other development programs, delay its potential commercialization or reduce the scope of any sales or marketing activities, or increase our expenditures and undertake development or commercialization activities at our own expense. If we elect to increase our expenditures to fund development or commercialization activities on our own, we may need to obtain additional capital, which may not be available to us on acceptable terms or at all. If we do not have sufficient funds, we may not be able to develop product candidates or bring them to market and generate product revenue.

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Risks Related to Our Intellectual Property

If we are unable to obtain and maintain patent and other intellectual property protection for SIG-001 or any other product candidates and for our SLTx platform, or if the scope of the patent and other intellectual property protection obtained is not sufficiently broad, our competitors could develop and commercialize products and technologies similar or identical to ours, and our ability to successfully commercialize any product candidates we may develop, and our SLTx platform may be adversely affected.

        Our commercial success will depend in large part on our ability to obtain and maintain patent, trademark, trade secret and other intellectual property protection of our SLTx platform technologies, product candidates and other technologies, methods used to manufacture them and methods of treatment, as well as successfully defending our patent and other intellectual property rights against third-party challenges. It is difficult, complex, time consuming and costly to protect cell-based technology, including our SLTx platform technologies. For example, important individual components of our platform and our product candidates may be in the prior art and available to third parties, and we may not be able to prevent use of such components in products that would compete with SIG-001 or our other product candidates. Our ability to stop unauthorized third parties from making, using, selling, offering to sell, importing or otherwise commercializing products similar to SIG-001 or any other product candidates is dependent upon the extent to which we have rights under valid and enforceable patents or trade secrets that cover these activities.

        We seek to protect our proprietary position by continuing to develop our own intellectual property and in-licensed intellectual property relating to our SLTx platform technologies and product candidates in the United States and abroad. If we or our licensors are unable to obtain or maintain patent protection with respect to our SLTx platform technologies and product candidates we may develop, or if the scope of the patent protection secured is not sufficiently broad, our competitors could develop and commercialize products and technologies similar or identical to ours and our ability to commercialize SIG-001 or any other product candidates may be adversely affected.

        The patent prosecution process is expensive, time-consuming, and complex, and we may not be able to file, prosecute, maintain, enforce, or license all necessary or desirable patent applications at a reasonable cost or in a timely manner in the United States and other important markets. In addition, we may not pursue or obtain patent protection in all relevant markets. It is also possible that we will fail to identify patentable aspects of our research and development output in time to obtain patent protection. Although we enter into non-disclosure and confidentiality agreements with parties who have access to confidential or patentable aspects of our research and development output, such as our employees, corporate collaborators, outside scientific collaborators, CROs, contract manufacturers, consultants, advisors, and other third parties, any of these parties may breach the agreements and disclose such output before a patent application is filed, thereby jeopardizing our ability to seek patent protection. In addition, our ability to obtain and maintain valid and enforceable patents depends, in part, on whether the differences between our inventions and the prior art allow our inventions to be patentable over the prior art. Furthermore, publications of discoveries in the scientific literature often lag behind the actual discoveries, and patent applications in the United States and other jurisdictions are typically not published until 18 months after filing, or in some cases not at all. Therefore, we cannot be certain that we or our licensors were the first to make the inventions claimed in our owned or any licensed patents or pending patent applications, or that we or our licensors were the first to file for patent protection of such inventions.

        The patent position of biotechnology and pharmaceutical companies generally is highly uncertain, involves complex legal and factual questions, and has been the subject of much litigation in recent years. The field of cell-based therapies has been the subject of extensive patenting activity. As a result, the issuance, scope, validity, enforceability, and commercial value of our patent rights are highly uncertain, and we may become involved in complex and costly litigation. Our pending and future patent applications may not result in patents being issued that protect our SLTx platform technologies or any of our current and

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future product candidates or that effectively prevent others from commercializing competitive technologies and product candidates.

        Moreover, the coverage claimed in a patent application can be significantly reduced before the patent is issued, and the scope of a patent claim may be reinterpreted after issuance. Even if our current or future owned and in-licensed patent applications issue as patents, the patents may not issue in a form that will provide us with any meaningful protection, prevent competitors or other third parties from competing with us, or otherwise provide us with any competitive advantage. Any patents that we own or in-license may be challenged, narrowed, circumvented, or invalidated by third parties. Consequently, we do not know whether any of our SLTx platform advances and any of our current and future product candidates will be protectable or remain protected by valid and enforceable patents. Our competitors or other third parties may be able to circumvent our patents by developing similar or alternative technologies or products in a non-infringing manner.

        In addition, given the amount of time required for the development, testing, and regulatory review of new product candidates, patents protecting such candidates might expire before or shortly after such candidates are commercialized. As a result, our intellectual property may not provide us with sufficient rights to exclude others from commercializing products similar or identical to ours. Any of the foregoing could have a material adverse effect on our competitive position, business, financial conditions, results of operations, and prospects.

Our rights to develop and commercialize our SLTx platform technologies and product candidates are subject, in part, to the terms and conditions of licenses granted to us by others.

        A significant portion of our intellectual property portfolio has been licensed from third parties, and our licensors may not always act in our best interest. If we fail to comply with our obligations under our intellectual property licenses, if the licenses are terminated, or if disputes regarding these licenses arise, we could lose significant rights that are important to our business.

        We have licensed and are dependent on certain patent rights and proprietary technology from third parties that are important or necessary to the development and commercialization of our technologies and product candidates. For example, we are a party to an exclusive patent license agreement with Massachusetts Institute of Technology, or MIT, pursuant to which we in-license key patents and patent applications co-owned by MIT and Boston Children's Hospital, or BCH, covering our SLTx platform technologies and product candidates. We refer to this agreement as the MIT License. The MIT License imposes various diligence, milestone payment, royalty, insurance, and other obligations on us. If we fail to comply with these obligations, MIT may have the right to terminate our license, in which event we may not be able to develop or market our SLTx platform or any other technologies or product candidates covered by the licensed intellectual property. In the future, we may also enter into additional license agreements that are material to the development or commercialization of our product candidates, and that may impose similar obligations as in the MIT License.

        These and other licenses may not provide sufficient rights to use such intellectual property, including cell lines or therapeutic protein sequences, in all relevant fields of use and in all territories in which we may wish to develop or commercialize our SLTx platform technologies and product candidates in the future. If we determine that rights to excluded fields or territories are necessary to commercialize our product candidates or maintain our competitive advantage, we may need to obtain additional licenses in order to continue developing, manufacturing or marketing our product candidates. We may not be able to obtain such licenses on an exclusive basis, on commercially reasonable terms, or at all, which could prevent us from commercializing our product candidates or allow our competitors or others the chance to access technology that is important to our business.

        We do not have complete control in the preparation, filing, prosecution, maintenance, enforcement, and defense of patents and patent applications covering the technology that we license from third parties.

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For example, pursuant to the MIT License, MIT retains control of preparation, filing, prosecution, and maintenance. We cannot be certain that these patents and patent applications will be prepared, filed, prosecuted, and maintained in a manner consistent with the best interests of our business. Also, in certain circumstances, MIT has the right to enforce and defend the licensed patents and patent applications. It is possible that any licensor enforcement of patents against infringers or defense of patents against challenges of validity or claims of enforceability may be less vigorous than if we had conducted them ourselves, or may not be conducted in accordance with our best interests. If we or our licensors fail to prosecute, maintain, enforce, and defend such patents, or if we or our licensors lose rights to those patents or patent applications, the rights we have licensed may be reduced or eliminated, our right to develop and commercialize SIG-001 and other potential product candidates that are the subject of such licensed rights could be adversely affected and we may not be able to prevent competitors from making, using, and selling competing products.

        Our licensors may not be the sole and exclusive owners or may not have sole and exclusive control of the patents, patent applications and technology we in-licensed. If other third parties have rights to any of such in-licensed intellectual property, they may be able to license such intellectual property to our competitors, and our competitors could market competing products and technology. In addition, our rights to our in-licensed patents, patent applications and technology are dependent, in part, on inter-institutional or other operating agreements between the joint owners of such intellectual property. If one or more of such joint owners breaches such inter-institutional or operating agreements, our rights to such in-licensed intellectual property may be adversely affected. Any of these events could have a material adverse effect on our competitive position, business, financial conditions, results of operations, and prospects.

        Furthermore, inventions contained within some of our in-licensed intellectual property, including patents and patent applications licensed from MIT, were made using funding from the U.S. government, and, in some cases, private, non-profit organizations. We rely on our licensors to ensure compliance with applicable obligations arising from such funding, such as timely reporting of the filing of patent applications arising out of the funded research and licenses granted to such patent applications. The failure of our licensors to meet their obligations may lead to a loss of rights to the relevant licensed intellectual property or the unenforceability of relevant patents.

        Also, university licensors, governments and other funding entities could have certain rights in our in-licensed patents and technology. For example, in the MIT License, MIT and BCH retain the right on behalf of themselves and all other non-profit research institutions to practice under the licensed patent rights for non-profit research, teaching and educational purposes, including sponsored research and collaborations, and the U.S. government retains a non-exclusive license authorizing the U.S. government to use the inventions or to have others use the invention on its behalf. If the U.S. government decides to exercise these rights, it is not required to engage us as its contractor in connection with doing so. The U.S. government's rights may also permit it to disclose the funded inventions and technology to third parties and to exercise march-in rights to use or allow third parties to use the technology we have licensed that was developed using U.S. government funding. The U.S. government may also exercise its march-in rights if it determines that action is necessary because we or our licensors failed to achieve practical application of the U.S. government-funded technology, because action is necessary to alleviate health or safety needs, to meet requirements of federal regulations, or to give preference to U.S. industry. In addition, our rights in such in-licensed U.S. government-funded inventions may be subject to certain requirements to manufacture product candidates embodying such inventions in the United States. Any of the foregoing could harm our business, financial condition, results of operations, and prospects significantly.

        In the event any of our third-party licensors determine that, in spite of our efforts, we have materially breached a license agreement or have failed to meet certain obligations thereunder, it may elect to terminate the applicable license agreement or, in some cases, one or more license(s) under the applicable license agreement and such termination could result in us no longer having the ability to develop and commercialize product candidates and technology covered by that license agreement or license. In the

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event of such termination of a third-party in-license, or if the underlying patents under a third-party in-license fail to provide the intended exclusivity, competitors could have the freedom to seek regulatory approval of, and to market, products identical to ours. Any of these events could have a material adverse effect on our competitive position, business, financial conditions, results of operations, and prospects.

Our owned patent applications and in-licensed patents and patent applications and other intellectual property may be subject to priority disputes or to inventorship disputes and similar proceedings.

        We or our licensors may be subject to claims that former employees, collaborators, or other third parties have an interest in our owned patent applications or in-licensed patents, patent applications, trade secrets or other intellectual property as an inventor or co-inventor. For example, we or our licensors may have inventorship disputes arise from conflicting obligations of employees, consultants or others who are involved in developing our current or any future product candidates. While it is our policy to require our employees and contractors who may be involved in the development of intellectual property to execute agreements assigning such intellectual property to us, we may be unsuccessful in executing such an agreement with each party who in fact develops intellectual property that we regard as our own. Litigation may be necessary to defend against these and other claims challenging inventorship or ownership of our owned patent applications, in-licensed patents or patent applications, trade secrets or other intellectual property. If we or our licensors are unsuccessful in defending any such claims or disputes, in addition to paying monetary damages, we may lose valuable intellectual property rights, such as exclusive ownership of, or the exclusive right to use, our owned or in-licensed patents or other intellectual property that is important to our current or any future product candidates. Even if we or our licensors are successful in defending against such claims, litigation could result in substantial costs and be a distraction to management and other employees. Any of the foregoing could result in a material adverse effect on our business, financial condition, results of operations, or prospects.

We have limited foreign intellectual property rights and may not be able to protect our intellectual property and proprietary rights throughout the world.

        We have limited intellectual property rights outside the United States. The process for obtaining patent protection outside the United States is particularly difficult, expensive, time consuming, and complex. Thus, filing, prosecuting, and defending patents on product candidates in all countries throughout the world would be prohibitively expensive, and our intellectual property rights in some countries outside the United States can be less extensive than those in the United States. In addition, the laws of foreign countries do not protect intellectual property rights to the same extent as federal and state laws of the United States. In addition, our intellectual property license agreements may not always include worldwide rights. Consequently, we may not be able to prevent third parties from practicing our owned and licensed inventions in all countries outside the United States, or from selling or importing products made using such inventions in and into the United States or other jurisdictions. Competitors may use our owned and licensed technologies in jurisdictions where we have not obtained patent protection, or in which our license rights are non-exclusive, to develop their own products and, further, may export otherwise infringing products to territories where we have patent protection but where enforcement is not as strong as that in the United States. These products may compete with our product candidates and our patents or other intellectual property rights may not be effective or sufficient to prevent them from competing.

        Many companies have encountered significant problems in protecting and defending intellectual property rights in foreign jurisdictions. The legal systems of certain countries, particularly certain developing countries, do not favor the enforcement of patents, trade secrets, and other intellectual property protection, particularly those relating to biotechnology and pharmaceutical products, which could make it difficult for us to stop the infringement of our patents or marketing of competing products against third parties in violation of our intellectual property and proprietary rights generally. Proceedings to enforce our patents and intellectual property rights in foreign jurisdictions could result in substantial costs

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and divert our efforts and attention from other aspects of our business, could put our patents at risk of being invalidated or interpreted narrowly and our patent applications at risk of not issuing, and could provoke third parties to assert claims against us. We may not prevail in any lawsuits that we initiate, and the damages or other remedies awarded, if any, may not be commercially meaningful. Moreover, the initiation of proceedings by third parties to challenge the scope or validity of our patent rights in foreign jurisdictions could result in substantial cost and divert our efforts and attention from other aspects of our business. Accordingly, our efforts to enforce our intellectual property and proprietary rights around the world may be inadequate to obtain a significant commercial advantage from the intellectual property that we develop or license.

        Many countries have compulsory licensing laws under which a patent owner may be compelled to grant licenses to third parties. In addition, many countries limit the enforceability of patents against government agencies or government contractors. In these countries, the patent owner may have limited remedies, which could materially diminish the value of such patent. If we or any of our licensors is forced to grant a license to third parties with respect to any patents relevant to our business, our competitive position may be impaired, and our business, financial condition, results of operations, and prospects may be adversely affected.

If we fail to comply with our obligations in the agreements under which we license intellectual property rights from third parties, or these agreements are terminated, or we otherwise experience disruptions to our business relationships with our licensors, we could lose license rights that are important to our business.

        We have entered into license agreements with third parties and may need to obtain additional licenses from our existing licensors and others to advance our research or allow commercialization of our product candidates. It is possible that we may be unable to obtain any additional licenses at a reasonable cost or on reasonable terms, if at all. In either event, we may be required to expend significant time and resources to redesign our technologies, product candidates, or the methods for manufacturing them or to develop or license replacement technologies, all of which may not be feasible on a technical or commercial basis. If we are unable to do so, we may be unable to develop or commercialize the affected product candidates, which could harm our business, financial condition, results of operations, and prospects significantly. We cannot provide any assurances that third-party patents do not exist which might be enforced against our current technology, manufacturing methods, product candidates, or future methods or products resulting in either an injunction prohibiting our manufacture or future sales, or, with respect to our future sales, an obligation on our part to pay royalties and/or other forms of compensation to third parties, which could be significant.

        In the MIT License, we have the first right to bring any actions against any third party for infringing on the patents we have exclusively licensed. Certain of our license agreements, including the MIT License, also require us to meet development thresholds to maintain the license, including establishing a set timeline for developing and commercializing products. In spite of our efforts, our licensors might conclude that we have materially breached our obligations under such license agreements and might therefore terminate the license agreements, thereby potentially removing or limiting our ability to develop and commercialize products and technologies covered by these license agreements. If these in-licenses are terminated, or if the underlying patents fail to provide the intended exclusivity, competitors or other third parties could have the freedom to seek regulatory approval of, and to market, products identical to ours and we may be required to cease our development and commercialization of our SLTx platform technologies or product candidates. Any of the foregoing could have a material adverse effect on our competitive position, business, financial conditions, results of operations, and growth prospects. Disputes may arise regarding intellectual property subject to a licensing agreement, including:

    the scope of rights granted under the license agreement and other interpretation-related issues;

    the extent to which our technologies and processes infringe on intellectual property of the licensor that is not subject to the licensing agreement;

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    the sublicensing of patent and other rights to third parties under our collaborative development relationships;

    our diligence obligations under the license agreement with respect to the use of the licensed technology in relation to our development and commercialization of our product candidates and what activities satisfy those diligence obligations;

    the inventorship and ownership of inventions and know-how resulting from the joint creation or use of intellectual property by our licensors and us and our partners;

    the effects of termination; and

    the priority of invention of patented technology.

        In addition, the agreements under which we currently license intellectual property or technology from third parties are complex, and certain provisions in such agreements may be susceptible to multiple interpretations. The resolution of any contract interpretation disagreement that may arise could narrow what we believe to be the scope of our rights to the relevant intellectual property or technology, or increase what we believe to be our financial or other obligations under the relevant agreement, either of which could have a material adverse effect on our business, financial condition, results of operations, and prospects. Moreover, if disputes over intellectual property that we have licensed prevent or impair our ability to maintain our current licensing arrangements on commercially acceptable terms, we may be unable to successfully develop and commercialize the affected product candidates. As a result, any termination of or disputes over our intellectual property licenses could result in the delay of our development and commercialization of our SLTx platform or other product candidates, the loss of our ability to develop and commercialize our SLTx platform or other product candidates, or our loss of other significant rights, any of which could have a material adverse effect on our business, financial conditions, results of operations, and prospects. It is also possible that a third party could be granted limited licenses to some of the same technology, in certain circumstances. For more information regarding our obligations in these agreements, please see "Business—License and Collaboration Agreements."

We may not be successful in acquiring or in-licensing necessary rights to key technologies or any product candidates we may develop.

        We currently have rights to intellectual property, through licenses from third parties, to identify and develop product candidates, and we may seek to in-license additional rights to key components of our SLTx platform. We may also seek to in-license rights to develop improvements to our SLTx platform or expand our product candidate pipeline. The future growth of our business may depend in part on our ability to in-license or otherwise acquire the rights to additional product candidates and technologies. Although we have succeeded in licensing technologies from third-party licensees including MIT in the past, we cannot assure you that we will be able to in-license or acquire the rights to any product candidates or technologies from third parties on acceptable terms or at all.

        We may enter into agreements with third-party licensors that provide that our field of use excludes particular fields. If we determine that rights to such fields are necessary to commercialize our product candidates or maintain our competitive advantage, we may need to obtain a license from such third parties in order to continue developing, manufacturing or marketing our product candidates. We may not be able to obtain such a license on an exclusive basis, on commercially reasonable terms, or at all, which could prevent us from commercializing our product candidates or allow our competitors or others the chance to access technology that is important to our business. For more information regarding these agreements, please see "Business—License and Collaboration Agreements."

        Furthermore, there has been extensive patenting activity in the fields of engineered cell therapy and encapsulated cell therapy, and pharmaceutical companies, biotechnology companies, and academic institutions are competing with us or are expected to compete with us in the field of cell therapy and filing

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patent applications potentially relevant to our business. Thus, there may be third-party patent applications, currently pending or filed in the future, that, if issued, may relate to our SLTx platform or product candidates. In order to market our product candidates, we may find it necessary or prudent to obtain licenses from such third-party intellectual property holders. However, we may be unable to secure such licenses or otherwise acquire or in-license any compositions, methods of use, processes, or other intellectual property rights from third parties that we identify as necessary for SIG-001 or any other product candidates. We may also require licenses from third parties for certain technologies related to preexisting cell therapies to be incorporated in our SLTx platform.

        Additionally, we may collaborate with academic institutions to accelerate our preclinical research or development under written agreements with these institutions. In certain cases, these institutions provide us with an option to negotiate a license to any of the institution's rights in technology resulting from the collaboration. Even if we hold such an option, we may be unable to negotiate a license from the institution within the specified timeframe or under terms that are acceptable to us. If we are unable to do so, the institution may offer the intellectual property rights to others, potentially blocking our ability to pursue our program.

        Furthermore, the research resulting in certain of our owned and in-licensed patent rights and technology may have been funded in part by the U.S. federal or state governments. As a result, the government may have certain rights, including march-in rights, to such patent rights and technology. When new technologies are developed with government funding, the government generally obtains certain rights in any resulting patents, including a non-exclusive license authorizing the government to use the invention for noncommercial purposes. These rights may permit the government to disclose our confidential information to third parties or allow third parties to use our licensed technology. The government can exercise its march-in rights if it determines that action is necessary because we fail to achieve practical application of the government-funded technology, because action is necessary to alleviate health or safety needs, to meet requirements of federal regulations, or to give preference to U.S. industry. In addition, our rights in such inventions may be subject to certain requirements to manufacture products embodying such inventions in the United States. Any of the foregoing could harm our competitive position, business, financial condition, results of operations and prospects.

        In addition, the licensing or acquisition of third-party intellectual property rights is a highly competitive area, and a number of more established companies are also pursuing strategies to license or acquire third-party intellectual property rights that we may consider attractive or necessary. These established companies may have a competitive advantage over us due to their size, capital resources and greater clinical development and commercialization capabilities. In addition, companies that perceive us to be a competitor may be unwilling to assign or license rights to us. We also may be unable to license or acquire third-party intellectual property rights on terms that would allow us to make an appropriate return on our investment or at all. If we are unable to successfully obtain rights to required third-party intellectual property rights or maintain the existing intellectual property rights we have, we may have to abandon development of the relevant program or product candidate, which could have a material adverse effect on our business, financial condition, results of operations, and prospects.

The biotechnology and pharmaceutical industries have experienced substantial litigation and other proceedings regarding intellectual property rights, and third parties may initiate legal proceedings alleging that we are infringing, misappropriating, or otherwise violating their intellectual property rights, the outcome of which would be uncertain and may prevent, delay or otherwise interfere with our product discovery and development efforts.

        Our commercial success depends upon our ability and the ability of our collaborators and licensors to develop, manufacture, market, and sell SIG-001 or any other product candidates and use our proprietary technologies without infringing, misappropriating, or otherwise violating the intellectual property and proprietary rights of third parties. The biotechnology and pharmaceutical industries are characterized by extensive litigation regarding patents and other intellectual property rights as well as administrative

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proceedings for challenging patents, including interference, derivation, inter partes review, post grant review, and reexamination proceedings before the USPTO or oppositions and other comparable proceedings in foreign jurisdictions. We may be subject to and may in the future become party to, or threatened with, adversarial proceedings or litigation regarding intellectual property rights with respect to our SLTx platform technologies and any product candidates we may develop, including interference proceedings, post-grant review, inter partes review, and derivation proceedings before the USPTO and similar proceedings in foreign jurisdictions such as oppositions before the EPO. Numerous U.S. and foreign issued patents and pending patent applications that are owned by third parties exist in the fields in which we are developing our product candidates and they may assert infringement claims against us based on existing patents or patents that may be granted in the future, regardless of their merit.

        As the biotechnology and pharmaceutical industries expand and more patents are issued, the risk increases that our SLTx platform technologies and product candidates may give rise to claims of infringement of the patent rights of others. Moreover, it is not always clear to industry participants, including us, which patents cover various types of therapies, products or their methods of use or manufacture. As with many technology-based products, there may be third-party patent applications that, if issued, may be construed to cover components of our SLTx platform and product candidates. There may also be third-party patents of which we are currently unaware with claims to technologies, compositions, methods of manufacture or methods of use.

        Because of the large number of patents issued and patent applications filed in our fields, third parties may allege they have patent rights encompassing our product candidates, technologies or methods. Third parties may assert that we are employing their proprietary technology without authorization and may file patent infringement claims or lawsuit against us, and if we are found to infringe such third-party patents, we may be required to pay damages, cease commercialization of the infringing technology, or obtain a license from such third parties, which may not be available on commercially reasonable terms or at all.

        Our ability to commercialize our product candidates in the United States and abroad may be adversely affected if we cannot obtain a license on commercially reasonable terms to relevant third-party patents that cover our product candidates or SLTx platform technologies. Even if we believe third-party intellectual property claims are without merit, there is no assurance that a court would find in our favor on questions of infringement, validity, enforceability, or priority. A court of competent jurisdiction could hold that these third-party patents are valid, enforceable, and infringed, which could materially and adversely affect our ability to commercialize SIG-001 or any other product candidates and any other product candidates or technologies covered by the asserted third-party patents. In order to successfully challenge the validity of any such U.S. patent in federal court, we would need to overcome a presumption of validity. As this burden is a high one requiring us to present clear and convincing evidence as to the invalidity of any such U.S. patent claims, there is no assurance that a court of competent jurisdiction would invalidate the claims of any such U.S. patent. If we are found to infringe a third party's intellectual property rights, and we are unsuccessful in demonstrating that such patents are invalid or unenforceable, we could be required to obtain a license from such third party to continue developing, manufacturing, and marketing SIG-001 or any other product candidates and our technologies. However, we may not be able to obtain any required license on commercially reasonable terms or at all. Even if we were able to obtain a license, it could be non-exclusive, thereby giving our competitors and other third parties access to the same technologies licensed to us, and it could require us to make substantial licensing and royalty payments. If we are unable to obtain a necessary license to a third-party patent on commercially reasonable terms, we may be unable to commercialize our SLTx platform technology or product candidates or such commercialization efforts may be significantly delayed, which could in turn significantly harm our business. We also could be forced, including by court order, to cease developing, manufacturing, and commercializing the infringing technology or product candidates. In addition, we could be found liable for significant monetary damages, including treble damages and attorneys' fees, if we are found to have willfully infringed a patent or other intellectual property right. Claims that we have misappropriated the confidential information or trade

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secrets of third parties could have a similar material adverse effect on our business, financial condition, results of operations, and prospects.

        Defense of third-party claims of infringement of misappropriation, or violation of intellectual property rights involves substantial litigation expense and would be a substantial diversion of management and employee time and resources from our business. Some third-parties may be able to sustain the costs of complex patent litigation more effectively than we can because they have substantially greater resources. In addition, any uncertainties resulting from the initiation and continuation of any litigation could have a material adverse effect on our ability to raise the funds necessary to continue our operations or could otherwise have a material adverse effect on our business, financial condition, results of operations and prospects. There could also be public announcements of the results of hearings, motions, or other interim proceedings or developments, and if securities analysts or investors perceive these results to be negative, it could have a substantial adverse effect on the price of our common stock. Any of the foregoing events could have a material adverse effect on our business, financial condition, results of operations and prospects.

We may become involved in lawsuits to protect or enforce our future patents or the patents of our licensors, which could be expensive, time consuming, and unsuccessful and could result in a finding that such patents are unenforceable or invalid.

        Competitors may infringe our future patents or the patents of our licensing partners, or we may be required to defend against claims of infringement. In addition, our future patents or the patents of our licensing partners also are, and may in the future become, involved in inventorship, priority, validity or enforceability disputes. Countering or defending against such claims can be expensive and time consuming. In an infringement proceeding, a court may decide that a patent owned or in-licensed by us is invalid or unenforceable, or may refuse to stop the other party from using the technology at issue on the grounds that our owned and in-licensed patents do not cover the technology in question. An adverse result in any litigation proceeding could put one or more of our owned or in-licensed patents at risk of being invalidated or interpreted narrowly.

        In patent litigation in the United States, defendant counterclaims alleging invalidity and/or unenforceability are commonplace, and there are numerous grounds upon which a third party can assert invalidity or unenforceability of a patent. Third parties may also raise similar claims before administrative bodies in the United States or abroad, even outside the context of litigation. These types of mechanisms include re-examination, post-grant review, inter partes review, interference proceedings, derivation proceedings, and equivalent proceedings in foreign jurisdictions (e.g., opposition proceedings). These types of proceedings could result in revocation or amendment to our patents such that they no longer cover our product candidates. The outcome for any particular patent following legal assertions of invalidity and unenforceability is unpredictable. With respect to the validity question, for example, we cannot be certain that there is no invalidating prior art, of which we, our licensors, our patent counsel and the patent examiner were unaware during prosecution. If a defendant were to prevail on a legal assertion of invalidity and/or unenforceability, or if we are otherwise unable to adequately protect our rights, we would lose at least part, and perhaps all, of the patent protection on our technology and/or product candidates. Defense of these types of claims, regardless of their merit, would involve substantial litigation expense and would be a substantial diversion of employee resources from our business.

        We may choose to challenge the patentability of claims in a third party's U.S. or foreign patent, regardless of whether the claims are a threat to our SLTx platform technologies or product candidates. In the United States, this may be done by requesting that the USPTO review the patent claims in re-examination, post-grant review, inter partes review, interference proceedings, derivation proceedings. There are equivalent proceedings in foreign jurisdictions (e.g., opposition proceedings). We are currently challenging a third-party patent in a patent opposition proceeding in the EPO, and in the future may choose to challenge third-party patents in the EPO and other foreign patent offices. Even if successful, the

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costs of these opposition proceedings could be substantial, and may consume our time or other resources. If we fail to obtain a favorable result at the USPTO, EPO or other patent office then we may be exposed to litigation by a third party alleging that the patent may be infringed by our product candidates, SLTx platform technologies or other proprietary technologies.

        Even if resolved in our favor, litigation or other legal proceedings relating to intellectual property claims may cause us to incur significant expenses and could distract our personnel from their normal responsibilities. Furthermore, because of the substantial amount of discovery required in connection with intellectual property litigation, there is a risk that some of our confidential information could be compromised by disclosure during this type of litigation. In addition, there could be public announcements of the results of hearings, motions, or other interim proceedings or developments, and if securities analysts or investors perceive these results to be negative, it could have a substantial adverse effect on the price of our common stock. Such litigation or proceedings could substantially increase our operating losses and reduce the resources available for development activities or any future sales, marketing, or distribution activities. We may not have sufficient financial or other resources to conduct such litigation or proceedings adequately. Some of our competitors may be able to sustain the costs of such litigation or proceedings more effectively than we can because of their greater financial resources and more mature and developed intellectual property portfolios. Uncertainties resulting from the initiation and continuation of patent litigation or other proceedings could have a material adverse effect on our ability to compete in the marketplace.

Obtaining and maintaining our patent protection depends on compliance with various procedural, document submission, fee payment, and other requirements imposed by government patent agencies, and our patent protection could be reduced or eliminated for non-compliance with these requirements.

        Periodic maintenance fees, renewal fees, annuity fees, and various other government fees on patents and applications are due to be paid to the USPTO and foreign patent agencies outside of the United States over the lifetime of our owned or licensed patents and applications. For our in-licensed patents and patent applications, we generally rely on our licensors, including MIT, to pay these fees due to U.S. and non-U.S. patent agencies. For our owned patent applications, we rely on our outside patent counsel in the United States and in foreign countries to monitor these deadlines and to pay these fees when so instructed.

        The USPTO and foreign patent agencies require compliance with several procedural, documentary, fee payment, and other similar provisions during the patent application process. We depend on our licensors to take the necessary action to comply with these requirements with respect to our licensed intellectual property, and for our owned patent applications, we engage counsel and other professionals to help us comply with these requirements. While an inadvertent lapse can be cured by payment of a late fee or by other means in accordance with the applicable rules, there are situations, however, in which non-compliance can result in a partial or complete loss of patent rights in the relevant jurisdiction. Were a noncompliance event to occur, our competitors might be able to enter the market with similar or identical products or technology, which could have a material adverse effect on our business, financial condition, results of operations, and prospects.

Changes in patent law in the United States and in non-U.S. jurisdictions could diminish the value of patents in general, thereby impairing our ability to protect our SLTx platform technologies and product candidates.

        As is the case with other biotech and pharmaceutical companies, our success is heavily dependent on intellectual property, particularly patents. Obtaining and enforcing patents in the biopharmaceutical industry involve both technological and legal complexity, and is therefore costly, time-consuming and inherently uncertain.

        Changes in either the patent laws or interpretation of the patent laws could increase the uncertainties and costs surrounding the prosecution of patent applications and the enforcement or defense of our issued

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patents. For example, in March 2013, under the Leahy-Smith America Invents Act, or the America Invents Act, the United States transitioned from a "first to invent" to a "first-to-file" patent system. Under a "first-to-file" system, assuming that other requirements for patentability are met, the first inventor to file a patent application generally will be entitled to a patent on an invention regardless of whether another inventor had made the invention earlier. A third party that files a patent application in the USPTO after March 2013, but before us could therefore be awarded a patent covering an invention of ours even if we had made the invention before it was made by such third party. This will require us to be cognizant going forward of the time from invention to filing of a patent application. Since patent applications in the United States and most other countries are confidential for a period of time after filing or until issuance, we cannot be certain that we or our licensors were the first to either file any patent application related to our technologies or product candidates or invent any of the inventions claimed in our or our licensor's patents or patent applications. The America Invents Act also includes a number of other significant changes to U.S. patent law, including provisions that affect the way patent applications will be prosecuted, allowing third-party submission of prior art and establishing a new post-grant review system, including post-grant review, inter partes review, and derivation proceedings. Because of a lower evidentiary standard in USPTO proceedings compared to the evidentiary standard in United States federal courts necessary to invalidate a patent claim, a third-party could potentially provide evidence in a USPTO proceeding sufficient for the USPTO to hold a claim invalid even though the same evidence would be insufficient to invalidate the claim if first presented in a district court action. Accordingly, a third-party may attempt to use the USPTO procedures to invalidate our patent claims that would not have been invalidated if first challenged by the third-party as a defendant in a district court action. The effects of these changes are currently unclear as the USPTO continues to promulgate new regulations and procedures in connection with the America Invents Act and many of the substantive changes to patent law, including the "first-to-file" provisions, only became effective in March 2013. In addition, the courts have yet to address many of these provisions and the applicability of the act and new regulations on the specific patents discussed in this filing have not been determined and would need to be reviewed. Therefore, the America Invents Act and its implementation could increase the uncertainties and costs surrounding the prosecution of our patent applications and the enforcement or defense of our issued patents.

        In addition, recent U.S. Supreme Court rulings have narrowed the scope of patent protection available in certain circumstances and weakened the rights of patent owners in certain situations. In addition to increasing uncertainty with regard to our ability to obtain patents in the future, this combination of events has created uncertainty with respect to the validity and enforceability of patents, once obtained. Depending on future actions by the U.S. Congress, the federal courts, and the USPTO, the laws and regulations governing patents could change in unpredictable ways that could weaken our ability to obtain new patents or to enforce our existing patents and patents that we might obtain in the future.

Patent terms may be inadequate to protect our competitive position on our product candidates for an adequate amount of time.

        Patents have a limited lifespan. The terms of individual patents depend upon the legal term for patents in the countries in which they are granted. In most countries, including the United States, if all maintenance fees are timely paid, the natural expiration of a patent is generally 20 years from its earliest non-provisional filing date in the applicable country. However, the actual protection afforded by a patent varies from country to country, and depends upon many factors, including the type of patent, the scope of its coverage, the availability of regulatory-related extensions, the availability of legal remedies in a particular country and the validity and enforceability of the patent. Various extensions including patent term extension, or PTE, and patent term adjustment, or PTA, may be available, but the life of such extension, and the protection they afford, is limited. Even if patents covering our product candidates are obtained, once the patent life has expired, we may be open to competition from competitive products, including biosimilars and generics. Given the amount of time required for the development, testing and regulatory review of new product candidates, patents protecting our product candidates might expire

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before or shortly after we or our partners commercialize those candidates. As a result, our owned and licensed patent portfolio may not provide us with sufficient rights to exclude others from commercializing products similar or identical to ours.

If we are unable to protect the confidentiality of our trade secrets, our business and competitive position would be harmed.

        In addition to seeking patents for our technologies and product candidates, we also rely on trade secret protection, as well as confidentiality agreements, non-disclosure agreements and invention assignment agreements with our employees, consultants and third-parties, to protect our know-how and other confidential and proprietary information, especially where we do not believe patent protection is appropriate or obtainable.

        It is our policy to require our employees, corporate collaborators, outside scientific collaborators, CROs, contract manufacturers, consultants, advisors, and other third parties to execute confidentiality agreements upon the commencement of employment or consulting relationships with us. These agreements provide that all confidential information concerning our business or financial affairs developed by or made known to the individual or entity during the course of the party's relationship with us is to be kept confidential and not disclosed to third parties, except in certain specified circumstances. In the case of employees, the agreements provide that all inventions conceived by the individual, and that are related to our current or planned business or research and development or made during normal working hours, on our premises or using our equipment or proprietary information, are our exclusive property. In the case of consultants and other third-party service providers, the agreements provide us with certain rights to all inventions arising from the services. However, we cannot guarantee that we have entered into such agreements with each party that may have or have had access to our trade secrets or proprietary technologies and processes. Additionally, the assignment of intellectual property rights may not be self-executing, or the assignment agreements may be breached, and we may be forced to bring claims against third parties, or defend claims that they may bring against us, to determine the ownership of what we regard as our intellectual property. Any of these parties may breach the agreements and disclose our proprietary information, including our trade secrets, and we may not be able to obtain adequate remedies for such breaches. Enforcing a claim that a party illegally disclosed or misappropriated a trade secret is difficult, expensive, and time-consuming, and the outcome is unpredictable.

        In addition to contractual measures, we try to protect the confidential nature of our proprietary information through other appropriate precautions, such as physical and technological security measures. However, trade secrets and know-how can be difficult to protect. These measures may not, for example, in the case of misappropriation of a trade secret by an employee or third-party with authorized access, provide adequate protection for our proprietary information. Our security measures may not prevent an employee or consultant from misappropriating our trade secrets and providing them to a competitor, and any recourse we might take against this type of misconduct may not provide an adequate remedy to protect our interests fully. In addition, our trade secrets may be independently developed by others in a manner that could prevent us from receiving legal recourse. If any of our confidential or proprietary information, such as our trade secrets, were to be disclosed or misappropriated, or if any of that information was independently developed by a competitor, our competitive position could be harmed.

        In addition, some courts inside and outside the United States are sometimes less willing or unwilling to protect trade secrets. If we choose to go to court to stop a third-party from using any of our trade secrets, we may incur substantial costs. Even if we are successful, these types of lawsuits may consume our time and other resources. Any of the foregoing could have a material adverse effect on our business, financial condition, results of operations and prospects.

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Third parties may assert that our employees, consultants, or advisors have wrongfully used or disclosed confidential information or misappropriated trade secrets.

        As is common in the biotechnology and pharmaceutical industries, we employ individuals that are currently or were previously employed at universities, research institutions or other biotechnology or pharmaceutical companies, including our competitors or potential competitors. Although we try to ensure that our employees, consultants, and advisors do not use the proprietary information or know-how of others in their work for us, we may be subject to claims that we or these individuals have inadvertently or otherwise used or disclosed intellectual property, including trade secrets or other proprietary information, of any such individual's current or former employer. We may then have to pursue litigation to defend against these claims. If we fail in defending any such claims, in addition to paying monetary damages, we may lose valuable intellectual property rights or personnel. Even if we are successful in defending against such claims, litigation could result in substantial costs and be a distraction to our technical and management personnel from their normal responsibilities. In addition, there could be public announcements of the results of hearings, motions or other interim proceedings or developments, and, if securities analysts or investors perceive these results to be negative, that perception could have a substantial adverse effect on the price of our common stock. This type of litigation or proceeding could substantially increase our operating losses and reduce our resources available for development activities, and we may not have sufficient financial or other resources to adequately conduct this type of litigation or proceedings. For example, some of our competitors may be able to sustain the costs of this type of litigation or proceedings more effectively than we can because of their substantially greater financial resources. In any case, uncertainties resulting from the initiation and continuation of intellectual property litigation or other intellectual property related proceedings could adversely affect our ability to compete in the marketplace.

If our trademarks and trade names are not adequately protected, then we may not be able to build name recognition in our markets of interest and our business may be adversely affected.

        Our registered or unregistered trademarks or trade names may be challenged, infringed, circumvented or declared generic or determined to be infringing on other marks. We may not be able to protect our rights to these trademarks and trade names, which we need to build name recognition among potential partners or customers in our markets of interest. At times, competitors or other third parties may adopt trade names or trademarks similar to ours, thereby impeding our ability to build brand identity and possibly leading to market confusion. In addition, there could be potential trade name or trademark infringement claims brought by owners of other registered trademarks or trademarks that incorporate variations of our registered or unregistered trademarks or trade names. Over the long term, if we are unable to establish name recognition based on our trademarks and trade names, then we may not be able to compete effectively and our business may be adversely affected. Our efforts to enforce or protect our proprietary rights related to trademarks, trade secrets, domain names, copyrights or other intellectual property may be ineffective and could result in substantial costs and diversion of resources and could adversely affect our business, financial condition, results of operations and growth prospects.

Intellectual property rights do not necessarily address all potential threats.

        The degree of future protection afforded by our intellectual property rights is uncertain because intellectual property rights have limitations and may not adequately protect our business or permit us to maintain our competitive advantage. For example:

    any of our current and future product candidates, if approved, will eventually become commercially available in generic or biosimilar product forms;

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    others may be able to make cell therapy products that are similar to any of our current and future product candidates or utilize similar cell therapy technology but that are not covered by the claims of the patents that we license or may own in the future;

    we, or our licensors or current or future collaborators, might not have been the first to make the inventions covered by the issued patent or pending patent application that we license or may own in the future;

    we, or our licensors or current or future collaborators, might not have been the first to file patent applications covering certain of our or their inventions;

    we, or our licensors or current or future collaborators, may fail to meet our obligations to the U.S. government regarding any in-licensed patents and patent applications funded by U.S. government grants, leading to the loss or unenforceability of patent rights;

    others may independently develop similar or alternative technologies or duplicate any of our technologies without infringing our owned or licensed intellectual property rights;

    it is possible that our pending, owned or licensed patent applications or those that we may own or license in the future will not lead to issued patents;

    it is possible that there are prior public disclosures that could invalidate our owned or in-licensed patents, or parts of our owned or in-licensed patents;

    it is possible that there are unpublished applications or patent applications maintained in secrecy that may later issue with claims covering our product candidates or technology similar to ours;

    it is possible that our owned or in-licensed patents or patent applications omit individual(s) that should be listed as inventor(s) or include individual(s) that should not be listed as inventor(s), which may cause these patents or patents issuing from these patent applications to be held invalid or unenforceable;

    issued patents that we hold rights to may be held invalid, unenforceable, or narrowed in scope, including as a result of legal challenges by our competitors;

    the claims of our owned or in-licensed issued patents or patent applications, if and when issued, may not cover our product candidates;

    the laws of foreign countries may not protect our proprietary rights or the proprietary rights of our licensors or current or future collaborators to the same extent as the laws of the United States;

    the inventors of our owned or in-licensed patents or patent applications may become involved with competitors, develop products or processes that design around our patents, or become hostile to us or the patents or patent applications on which they are named as inventors;

    our competitors might conduct research and development activities in countries where we do not have patent rights and then use the information learned from such activities to develop competitive products for sale in our major commercial markets;

    we have engaged in scientific collaborations in the past and will continue to do so in the future and our collaborators may develop adjacent or competing products that are outside the scope of our patents;

    we may not develop additional proprietary technologies that are patentable;

    any product candidates we develop may be covered by third parties' patents or other exclusive rights;

    the patents of others may harm our business; or

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    we may choose not to file a patent in order to maintain certain trade secrets or know-how, and a third-party may subsequently file a patent covering such intellectual property.

        Should any of these events occur, they could have a material adverse effect on our business, financial condition, results of operations, and prospects.

Risks Related to Regulatory and Compliance Matters

Our relationships with healthcare providers, physicians, and third-party payors will be subject to applicable anti-kickback, fraud and abuse, anti-bribery and other healthcare laws and regulations, which could expose us to criminal sanctions, civil penalties, contractual damages, reputational harm, and diminished profits and future earnings.

        Physicians, other healthcare providers and third-party payors will play a primary role in the recommendation and prescription of SIG-001 or any other product candidates for which we obtain marketing approval. Our future arrangements with third-party payors and customers may expose us to broadly applicable fraud and abuse and other healthcare laws and regulations that may constrain the business or financial arrangements and relationships through which we market, sell, and distribute our medicines for which we obtain marketing approval. Restrictions under applicable federal and state healthcare laws and regulations include the following:

    federal false claims, false statements and civil monetary penalties laws prohibiting, among other things, any person from knowingly presenting, or causing to be presented, a false claim for payment of government funds or knowingly making, or causing to be made, a false statement to get a false claim paid;

    the federal Anti-Kickback Statute, which prohibits, among other things, persons from offering, soliciting, receiving or providing remuneration, directly or indirectly, to induce either the referral of an individual, or the purchasing or ordering of, a good or service for which payment may be made under federal healthcare programs such as Medicare and Medicaid. A person or entity does not need to have actual knowledge of this statute or specific intent to violate it in order to have committed a violation. In addition, the government may assert that a claim including items or services resulting from a violation of the federal Anti-Kickback Statute constitutes a false or fraudulent claim for purposes of the federal civil False Claims Act;

    the federal Health Insurance Portability and Accountability Act of 1996, or HIPAA, which, in addition to privacy protections applicable to healthcare providers and other entities, prohibits executing a scheme to defraud any healthcare benefit program or making false statements relating to healthcare matters. Similar to the federal Anti-Kickback Statute, a person or entity does not need to have actual knowledge of the statute or specific intent to violate it in order to have committed a violation;

    the federal Food, Drug, and Cosmetic Act, or the FDCA, which among other things, strictly regulates pharmaceutical marketing, prohibits manufacturers from marketing such products for off-label use and regulates the distribution of samples;

    federal laws that require pharmaceutical manufacturers to report certain calculated product prices to the government or provide certain discounts or rebates to government authorities or private entities, often as a condition of reimbursement under government healthcare programs;

    the so-called "federal sunshine" law under the Healthcare Reform Act, which requires pharmaceutical and medical device companies to monitor and report certain financial interactions with certain healthcare providers to the Center for Medicare & Medicaid Services within the U.S. Department of Health and Human Services for re-disclosure to the public, as well as ownership and investment interests held by physicians and their immediate family members; and

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    analogous state and foreign laws and regulations, such as state anti-kickback, anti-bribery and false claims laws, which may apply to healthcare items or services that are reimbursed by non-governmental third-party payors, including private insurers.

        Some state laws also require pharmaceutical companies to comply with specific compliance standards, restrict financial interactions between pharmaceutical companies and healthcare providers or require pharmaceutical companies to report information related to payments to health care providers or marketing expenditures.

        Efforts to ensure that our business arrangements with third parties will comply with applicable healthcare laws and regulations will involve substantial costs. Given the breadth of the laws and regulations, limited guidance for certain laws and regulations and evolving government interpretations of the laws and regulations, governmental authorities may possibly conclude that our business practices may not comply with healthcare laws and regulations. If our operations are found to be in violation of any of the laws described above or any other government regulations that apply to us, we may be subject to penalties, including civil and criminal penalties, damages, fines, exclusion from participation in government health care programs, such as Medicare and Medicaid, imprisonment, and the curtailment or restructuring of our operations, any of which could adversely affect our business, financial condition, results of operations, and prospects.

        When carrying out any activity or inducement within the U.K. or EU designed to promote the prescription, supply, sale or consumption of medicinal products to persons qualified to prescribe or supply them (including, for example, physicians), no gifts, pecuniary advantages or benefits in kind may be supplied, offered or promised to such persons unless they are inexpensive and relevant to the practice of medicine or pharmacy. The provision of benefits or advantages to such individuals more generally is also governed by the national anti-bribery laws of the U.K. and the EU member states, such as the U.K. Bribery Act 2010. Infringement of these laws could result in substantial fines and imprisonment, or in being excluded from public tenders for our products.

        Payments made by biopharmaceutical companies to healthcare organizations, healthcare professionals (including physicians) and patient organizations in the U.K. and EU are required to be publicly disclosed. Direct and indirect payments and transfers of value are caught, including donations, grants, sponsorships, hospitality, fees for research and development, consultancy services and gifts. Moreover, in some EU Members States, agreements with physicians often must be the subject of prior notification and approval by the physician's employer, his or her competent professional organization, and/or the relevant regulatory authorities. These requirements are provided in the national laws, industry codes, or professional codes of conduct applicable in the U.K. and EU member states. Failure to comply with these requirements could result in reputational risk, public reprimands, administrative penalties, fines or imprisonment.

Healthcare and other reform legislation may increase the difficulty and cost for us and any collaborators we may have to obtain marketing approval of and commercialize SIG-001 or any other product candidates and affect the prices we, or they, may obtain.

        In the United States and some foreign jurisdictions, there have been and continue to be ongoing efforts to implement legislative and regulatory changes regarding the healthcare system. Such changes could prevent or delay marketing approval of any product candidates that we may develop, restrict or regulate post-approval activities, and affect our ability to profitably sell any product candidates for which we obtain marketing approval. Although we cannot predict what healthcare or other reform efforts will be successful, such efforts may result in more rigorous coverage criteria, in additional downward pressure on the price that we, or our future collaborators, may receive for any approved products or in other consequences that may adversely affect our ability to achieve or maintain profitability.

        In the United States, the federal government and individual states have aggressively pursued healthcare reform, as evidenced by the passing of the Affordable Care Act and the ongoing efforts to

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modify or repeal that legislation. The Affordable Care Act substantially changed the way healthcare is financed by both governmental and private insurers and contains a number of provisions that affect coverage and reimbursement of drug products and/or that could potentially reduce the demand for pharmaceutical products such as increasing drug rebates under state Medicaid programs for brand name prescription drugs and extending those rebates to Medicaid managed care and assessing a fee on manufacturers and importers of brand name prescription drugs reimbursed under certain government programs, including Medicare and Medicaid. Other aspects of healthcare reform, such as expanded government enforcement authority and heightened standards that could increase compliance-related costs, could also affect our business. Modifications have been implemented under the Trump Administration and additional modifications or repeal may occur. There are, and may continue to be, judicial challenges. See "Government Regulation—Healthcare and Other Reform." We cannot predict the ultimate content, timing or effect of any changes to the Affordable Care Act or other federal and state reform efforts. There is no assurance that federal or state health care reform will not adversely affect our future business and financial results, and we cannot predict how future federal or state legislative, judicial or administrative changes relating to healthcare reform will affect our business.

        Federal and state governments have shown significant interest in implementing cost-containment programs to limit the growth of government-paid healthcare costs, including price controls, waivers from Medicaid drug rebate law requirements, restrictions on reimbursement and requirements for substitution of generic products for branded prescription drugs. The private sector has also sought to control healthcare costs by limiting coverage or reimbursement or requiring discounts and rebates on products. We are unable to predict what additional legislation, regulations or policies, if any, relating to the healthcare industry or third-party coverage and reimbursement may be enacted in the future or what effect such legislation, regulations or policies would have on our business. Any cost containment measures could significantly decrease the available coverage and the price we might establish for our products, which would have an adverse effect on our net revenues and operating results.

        Legislative and regulatory proposals have been made to expand post-approval requirements and restrict sales and promotional activities for biotechnology products. We cannot be sure whether additional legislative changes will be enacted, or whether FDA regulations, guidance or interpretations for biological products will be changed, or what the impact of such changes on the marketing approvals of our product candidates, if any, may be. In addition, increased scrutiny by the U.S. Congress of the FDA's approval and decision-making processes may significantly delay or prevent marketing approval, as well as subject us to more stringent product labeling and post-marketing testing and other requirements.

We have obtained orphan drug designation for SIG-001 for the treatment of Hemophilia Type A, and we intend to seek orphan drug designation for our product candidates, but any orphan drug designations we receive may not confer marketing exclusivity or other expected benefits.

        Under the Orphan Drug Act, the FDA may designate a product as an orphan product if it is intended to treat a rare disease or condition, which is generally defined as a patient population of fewer than 200,000 individuals in the United States, or a patient population of greater than 200,000 individuals in the United States, but for which there is no reasonable expectation that the cost of developing the drug or biologic will be recovered from sales in the United States. A similar regulatory scheme governs approval of orphan product candidates by the EMA in the EU. In the United States, orphan designation entitles a party to financial incentives such as opportunities for grant funding towards clinical trial costs, tax advantages and user fee waivers. In addition, if a product with an orphan drug designation subsequently receives the first marketing approval for the indication for which it has such designation, the product is entitled to a period of orphan drug exclusivity. Orphan drug exclusivity in the United States provides that the FDA may not approve any other applications, including a full BLA, to market the same biologic for the same indication for seven years, except in limited circumstances. The applicable exclusivity period is ten years in Europe. The exclusivity period in the EU can be reduced to six years if a product no longer meets

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the criteria for orphan drug designation, in particular if the product is sufficiently profitable so that market exclusivity is no longer justified. We were granted Orphan Drug designation for SIG-001 for the treatment of Hemophilia A by the FDA in August 2019 and by the EMA in November 2020. However, we may not be able to obtain orphan drug designation for our other product candidates, and previously granted orphan drug designations may be revoked. Any product candidates we may develop for prevalent diseases, such as diabetes, will not be eligible to receive orphan drug designation.

        Even if we obtain U.S. orphan drug exclusivity for a product candidate, that exclusivity may not effectively protect the product candidate from competition because different product candidates can be approved for the same condition. In addition, even after an orphan drug is approved, the FDA can subsequently approve the same product candidate for the same condition if the FDA concludes that the later product candidate is clinically superior in that it is shown to be safer, more effective or makes a major contribution to patient care compared with the product that has orphan exclusivity. The EU has its own criteria for designation as an orphan medicine but, as in the United States, orphan market exclusivity may not apply to the extent any further applicant can establish that its medicinal product is safer, more effective or otherwise clinically superior. Orphan drug exclusivity in the United States or the EU may also be lost if the FDA or EMA, respectively, determines that the request for designation was materially defective or if the manufacturer is unable to assure sufficient quantity of the product to meet the needs of the patients with the rare disease or condition.

        On August 3, 2017, the Congress passed the FDA Reauthorization Act of 2017, or FDARA. FDARA, among other things, codified the FDA's pre-existing regulatory interpretation, to require that a drug sponsor demonstrate the clinical superiority of an orphan drug that is otherwise the same as a previously approved drug for the same rare disease in order to receive orphan drug exclusivity. The new legislation reverses prior precedent holding that the Orphan Drug Act unambiguously requires that the FDA recognize the orphan exclusivity period regardless of a showing of clinical superiority. The FDA may further reevaluate the Orphan Drug Act and its regulations and policies. We do not know if, when, or how the FDA may change the orphan drug regulations and policies in the future, and it is uncertain how any changes might affect our business. Depending on what changes the FDA may make to its orphan drug regulations and policies, our business could be adversely impacted.

Our employees, principal investigators, consultants, and commercial partners may engage in misconduct or other improper activities, including non-compliance with regulatory standards and requirements and insider trading.

        We are exposed to the risk of fraud or other misconduct by our employees, consultants, and commercial partners, and, if we commence clinical trials, our principal investigators. Misconduct by these parties could include intentional failures to comply with FDA regulations or the regulations applicable in the EU and other jurisdictions, provide accurate information to the FDA, the EMA, and other regulatory authorities, comply with healthcare fraud and abuse laws and regulations in the United States and abroad, report financial information or data accurately, or disclose unauthorized activities to us. In particular, sales, marketing, and business arrangements in the healthcare industry are subject to extensive laws and regulations intended to prevent fraud, misconduct, kickbacks, self-dealing and other abusive practices. These laws and regulations restrict or prohibit a wide range of pricing, discounting, marketing and promotion, sales commission, customer incentive programs, and other business arrangements. Such misconduct also could involve the improper use of information obtained in the course of clinical trials or interactions with the FDA, the EMA or other regulatory authorities, which could result in regulatory sanctions and cause serious harm to our reputation. It is not always possible to identify and deter employee misconduct, and the precautions we take to detect and prevent this activity may not be effective in controlling unknown or unmanaged risks or losses or in protecting us from government investigations or other actions or lawsuits stemming from a failure to comply with these laws or regulations. If any such actions are instituted against us, and we are not successful in defending ourselves or asserting our rights,

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those actions could have a significant impact on our business, financial condition, results of operations, and prospects, including the imposition of significant fines or other sanctions.

Laws and regulations governing any international operations we may have in the future may preclude us from developing, manufacturing and selling certain product candidates outside of the United States and require us to develop and implement costly compliance programs.

        We are subject to numerous laws and regulations in each jurisdiction outside the United States in which we operate. The creation, implementation and maintenance of international business practices compliance programs is costly and such programs are difficult to enforce, particularly where reliance on third parties is required.

        The Foreign Corrupt Practices Act, or FCPA, prohibits any U.S. individual or business from paying, offering, authorizing payment or offering of anything of value, directly or indirectly, to any foreign official, political party or candidate for the purpose of influencing any act or decision of the foreign entity in order to assist the individual or business in obtaining or retaining business. The FCPA also obligates companies whose securities are listed in the United States to comply with certain accounting provisions requiring the company to maintain books and records that accurately and fairly reflect all transactions of the corporation, including international subsidiaries, and to devise and maintain an adequate system of internal accounting controls for international operations. The anti-bribery provisions of the FCPA are enforced primarily by the Department of Justice. The SEC is involved with enforcement of the books and records provisions of the FCPA.

        Similarly, the U.K. Bribery Act 2010 has extra-territorial effect for companies and individuals having a connection with the United Kingdom. The U.K. Bribery Act prohibits inducements both to public officials and private individuals and organizations. Compliance with the FCPA and the U.K. Bribery Act is expensive and difficult, particularly in countries in which corruption is a recognized problem. In addition, the FCPA presents particular challenges in the pharmaceutical industry, because, in many countries, hospitals are operated by the government, and doctors and other hospital employees are considered foreign officials. Certain payments to hospitals in connection with clinical trials and other work have been deemed to be improper payments to government officials and have led to FCPA enforcement actions.

        Various laws, regulations and executive orders also restrict the use and dissemination outside of the United States, or the sharing with certain non-U.S. nationals, of information classified for national security purposes, as well as certain products and technical data relating to those products. If we expand our business outside of the United States, we will be required to dedicate additional resources to comply with these laws, and these laws may preclude us from developing, manufacturing, or selling certain products and product candidates outside of the United States, which could limit our growth potential and increase our development costs. The failure to comply with laws governing international business practices may result in substantial penalties, including suspension or debarment from government contracting. Violation of the FCPA can result in significant civil and criminal penalties. Indictment alone under the FCPA can lead to suspension of the right to do business with the U.S. government until the pending claims are resolved. Conviction of a violation of the FCPA can result in long-term disqualification as a government contractor. The termination of a government contract or relationship as a result of our failure to satisfy any of our obligations under laws governing international business practices would have a negative impact on our operations and harm our reputation and ability to procure government contracts. The SEC also may suspend or bar issuers from trading securities on U.S. exchanges for violations of the FCPA's accounting provisions.

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We are subject to stringent privacy laws, information security laws, regulations, policies and contractual obligations related to data privacy and security and changes in such laws, regulations, policies and contractual obligations could adversely affect our business.

        We are subject to data privacy and protection laws and regulations that apply to the collection, transmission, storage and use of personally-identifying information, which among other things, impose certain requirements relating to the privacy, security and transmission of personal information, including comprehensive regulatory systems in the United States and EU. The legislative and regulatory landscape for privacy and data protection continues to evolve in jurisdictions worldwide, and there has been an increasing focus on privacy and data protection issues with the potential to affect our business. We cannot be sure how these evolving laws and regulations will be interpreted, enforced or applied to our operations. Failure to comply with any of these laws and regulations could result in contractual liabilities as well as enforcement action against us. As a result, we could be subject to fines, claims for damages by affected individuals, negative publicity, damage to our reputation and loss of goodwill, any of which could have a material adverse effect on our business, financial condition, results of operations or prospects. Even if we are not determined to have violated these laws, government investigations into these issues typically require the expenditure of significant resources and generate negative publicity, which could harm our business, financial condition, results of operations or prospects. Applicable privacy laws and court decisions in the EU could also impact our ability to transfer personal data internationally.

        Within the United States, there are numerous federal and state laws and regulations related to the privacy and security of personal information. In particular, regulations promulgated pursuant to HIPAA establish privacy and security standards that limit the use and disclosure of personally identifiable health information, or protected health information, and require the implementation of administrative, physical and technological safeguards to protect the privacy of protected health information and ensure the confidentiality, integrity and availability of electronic protected health information. Determining whether protected information has been handled in compliance with the various applicable requirements and our contractual obligations can be complex and may be subject to changing interpretation.

        Additionally, the California Consumer Privacy Act, or the CCPA, became effective on January 1, 2020 with enforcement beginning July 1, 2020. The CCPA imposes stringent data privacy and data protection requirements for the data of California residents. Among other things, it requires covered companies to provide new disclosures to California consumers and afford such consumers new data protection rights, including the ability to opt-out of certain sales of personal information. The CCPA provides for civil penalties for violations, as well as a private right of action for certain data breaches that result in the loss of personal data that may increase the likelihood of, and risks associated with, data breach litigation. The effects of this legislation are potentially far-reaching and may require us to modify our data processing practices and policies and to incur substantial costs and expenses in an effort to comply.

        We have initiated our Phase 1/2 clinical trial for SIG-001 in the United Kingdom, and data collected from patients enrolled in this study is subject to the General Data Protection Regulation, or GDPR, which went into effect in May 2018 and which imposes obligations on companies that operate in our industry with respect to the processing of personal data and the cross-border transfer of such data. The GDPR imposes onerous accountability obligations requiring data controllers and processors to maintain a record of their data processing and policies. If our or our partners' or service providers' privacy or data security measures fail to comply with the GDPR requirements, we may be subject to litigation, regulatory investigations, enforcement notices requiring us to change the way we use personal data and/or fines of up to 20 million Euros or up to 4% of the total worldwide annual turnover of the preceding financial year, whichever is higher, as well as compensation claims by affected individuals, negative publicity, reputational harm and a potential loss of business and goodwill. Additionally, on July 16, 2020 the CJEU, Europe's highest court, held in the Schrems II case that the EU-US Privacy Shield, a mechanism for the transfer of personal data from the EU to the United States, was invalid. The impact of this decision on the ability to lawfully transfer personal information from the EU to the United States is being assessed and guidance from European

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regulators and advisor bodies is awaited. It is possible that the decision will restrict the ability to transfer personal data from the EU to the United States.

        Data privacy regulations and data privacy remain an evolving landscape at both the domestic and international level, with new regulations coming into effect, such as the California Consumer Privacy Act, and continued legal challenges, and our efforts to comply with the evolving data protection rules may be unsuccessful. It is possible that these laws may be interpreted and applied in a manner that is inconsistent with our practices. We must devote significant resources to understanding and complying with this changing landscape and such changes may require ongoing modifications to our policies, procedures and systems.

Risks Related to Employee and Operations Matters, Managing Growth and Information Technology

Our future success depends on our ability to retain our Chief Executive Officer and other key executives and to attract, retain, and motivate qualified personnel.

        We are highly dependent on Dr. Rogerio Vivaldi Coelho, our Chief Executive Officer, as well as the other principal members of our management and scientific teams. Dr. Vivaldi and such other principal members are employed "at will," meaning we or they may terminate the employment at any time. We do not maintain "key person" insurance for any of our executives or other employees. The loss of the services of any of these persons could impede the achievement of our research, development, and commercialization objectives.

        Recruiting and retaining qualified scientific, clinical, manufacturing, business development, general and administrative and sales and marketing personnel will also be critical to our success. We may not be able to attract and retain these personnel on acceptable terms given the competition among numerous pharmaceutical and biotechnology companies for similar personnel. We also experience competition for the hiring of scientific and clinical personnel from universities and research institutions. In addition, we rely on consultants and advisors, including scientific and clinical advisors, to assist us in formulating our research and development and commercialization strategy. Our consultants and advisors, including our scientific co-founders, may be employed by employers other than us and may have commitments under consulting or advisory contracts with other entities that may limit their availability to us. The inability to recruit, or loss of services of certain executives, key employees, consultants, or advisors, may impede the progress of our research, development, and commercialization objectives and have a material adverse effect on our business, financial condition, results of operations, and prospects.

We expect to expand our development, regulatory, and future sales and marketing capabilities, and as a result, we may encounter difficulties in managing our growth, which could disrupt our operations.

        As of September 30, 2020, we had 96 full-time employees and, in connection with the growth and advancement of our pipeline and becoming a public company, we expect to increase the number of our employees and the scope of our operations, particularly in the areas of product development, regulatory affairs, and sales and marketing. To manage our anticipated future growth, we must continue to implement and improve our managerial, operational, and financial systems, expand our facilities, and continue to recruit and train additional qualified personnel. Due to our limited financial resources and the limited experience of our management team in managing a company with such anticipated growth, we may not be able to effectively manage the expected expansion of our operations or recruit and train additional qualified personnel. Moreover, the expected physical expansion of our operations may lead to significant costs and may divert our management and business development resources. Any inability to manage growth could delay the execution of our business plans or disrupt our operations.

        As a growing biotechnology company, we are actively pursuing new platforms and product candidates in many therapeutic areas and across a wide range of diseases. Successfully developing product candidates for and fully understanding the regulatory and manufacturing pathways to all of these therapeutic areas

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and disease states requires a significant depth of talent, resources and corporate processes in order to allow simultaneous execution across multiple areas. Due to our limited resources, we may not be able to effectively manage this simultaneous execution and the expansion of our operations or recruit and train additional qualified personnel. This may result in weaknesses in our infrastructure, give rise to operational mistakes, legal or regulatory compliance failures, loss of business opportunities, loss of employees and reduced productivity among remaining employees. The physical expansion of our operations may lead to significant costs and may divert financial resources from other projects, such as the development of our product candidates. If our management is unable to effectively manage our expected development and expansion, our expenses may increase more than expected, our ability to generate or increase our revenue could be reduced and we may not be able to implement our business strategy. Our future financial performance and our ability to compete effectively and commercialize our product candidates, if approved, will depend in part on our ability to effectively manage the future development and expansion of our company.

Our internal computer systems, or those of our third-party vendors, collaborators or other contractors or consultants, may fail or suffer security breaches, which could result in a material disruption of our product development programs, compromise sensitive information related to our business or prevent us from accessing critical information, potentially exposing us to liability or otherwise adversely affecting our business.

        Our internal computer systems and those of our current and any future third-party vendors, collaborators and other contractors or consultants are vulnerable to damage, interruption or data theft from computer viruses, computer hackers, malicious code, employee theft or misuse, ransomware, social engineering (including phishing attacks), denial-of-service attacks, sophisticated nation-state and nation-state-supported actors, unauthorized access, natural disasters, terrorism, war and telecommunication and electrical failures. Cybersecurity incidents, which may not be immediately or ever detected, are increasing in frequency and evolving in nature.

        While we seek to protect our information technology systems from system failure, accident and security breach, if such an event were to occur and cause interruptions in our operations, it could result in a disruption of our development programs and our business operations, whether due to a loss of our trade secrets or other proprietary information or other disruptions. For example, the loss of clinical trial data from future clinical trials could result in delays in our regulatory approval efforts and significantly increase our costs to recover or reproduce the data. If we were to experience a significant cybersecurity breach of our information systems or data, the costs associated with the investigation, remediation and potential notification of the breach to counter-parties and data subjects could be material. In addition, our remediation efforts may not be successful. If we do not allocate and effectively manage the resources necessary to build and sustain the proper technology and cybersecurity infrastructure, we could suffer significant business disruption, including transaction errors, supply chain or manufacturing interruptions, processing inefficiencies, data loss or the loss of or damage to intellectual property or other proprietary information. In addition, in response to the ongoing COVID-19 pandemic, a majority of our workforce is currently working remotely. This could increase our cyber security risk, create data accessibility concerns, and make us more susceptible to communication disruption.

        To the extent that any disruption or security breach were to result in a loss of, or damage to, our or our third-party vendors', collaborators' or other contractors' or consultants' data or applications, or inappropriate disclosure of confidential or proprietary information, we could incur liability including litigation exposure, penalties and fines, we could become the subject of regulatory action or investigation, our competitive position could be harmed and the further development and commercialization of our product candidates could be delayed. Any of the above could have a material adverse effect on our business, financial condition, results of operations or prospects. While we maintain cyber-liability insurance (covering security and privacy matters), such insurance may not be adequate to cover any losses experienced as a result of a cybersecurity incident.

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A pandemic, epidemic, or outbreak of an infectious disease, such the COVID-19 pandemic, may materially and adversely affect our business and our financial results and could cause a disruption to the development or supply of SIG-001 or any other product candidates.

        Public health crises such as pandemics or similar outbreaks could adversely impact our business. Recently, a novel strain of a virus named SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2), or coronavirus, which causes COVID-19, has spread to most countries across the world, including all 50 states within the United States, and including specifically Cambridge, Massachusetts, where our primary office is located. The COVID-19 pandemic is evolving, and to date has led to the implementation of various responses, including government-imposed quarantines, travel restrictions and other public health safety measures. The extent to which the coronavirus impacts our operations or those of our collaborators will depend on future developments, which are highly uncertain and cannot be predicted with confidence, including the duration of the outbreak, new information that will emerge concerning the severity of the coronavirus and the actions to contain the coronavirus or treat its impact, among others.

        The continued spread of COVID-19 globally could adversely impact any preclinical or clinical trial operations in the United States and Europe, including our ability to recruit and retain patients and principal investigators and site staff who, as healthcare providers, may have heightened exposure to COVID-19 if an outbreak occurs in their geography, and our ability to conduct preclinical studies with reduced laboratory capacity. For example, similar to other biotechnology companies, we have, and may in the future, experience delays in initiating IND-enabling studies, protocol deviations, enrolling in any clinical trials or dosing of patients in any clinical trials as well as in activating any trial sites.

        In addition, the patient populations that SIG-001 or any other product candidates target may be particularly susceptible to COVID-19, which may make it more difficult for us to identify patients able to enroll in our current and future clinical trials and may impact the ability of enrolled patients to complete any such trials. Any negative impact the COVID-19 pandemic has to patient enrollment or treatment or the execution of SIG-001 or any other product candidates could cause costly delays to clinical trial activities, which could adversely affect our ability to obtain regulatory approval for and to commercialize our product candidates, increase our operating expenses and have a material adverse effect on our financial results. Additionally, timely enrollment in planned clinical trials is dependent upon clinical trial sites which could be adversely affected by global health matters, such as pandemics. We plan to conduct clinical trials for SIG-001 and any other product candidates in geographies which are currently being affected by the COVID-19 pandemic. Some factors from the coronavirus outbreak that will delay or otherwise adversely affect future enrollment in the clinical trials of SIG-001 or any other product candidates, as well as our business generally, include:

    delays in receiving approval from local regulatory authorities to initiate our planned clinical trials;

    delays or difficulties in enrolling patients in our clinical trials;

    delays or difficulties in clinical site initiation, including difficulties in recruiting clinical site investigators and clinical site staff;

    changes in local regulations as part of a response to the COVID-19 pandemic which may require us to change the ways in which our clinical trials are conducted, which may result in unexpected costs, or cause us to discontinue such clinical trials altogether;

    refusal of the FDA or other regulatory authorities to accept data from clinical trials gathered in affected geographies;

    the risk that participants enrolled in our clinical trials will contract COVID-19 while the clinical trial is ongoing, which could impact the results of the clinical trial, including by increasing the number of observed adverse events;

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    the potential diversion of healthcare resources away from the conduct of clinical trials to focus on pandemic concerns, including the attention of physicians serving as our clinical trial investigators, hospitals serving as our clinical trial sites and hospital staff supporting the conduct of our prospective clinical trials;

    limitations on travel that could interrupt key trial and business activities, such as clinical trial site initiations and monitoring, domestic and international travel by employees, contractors or patients to clinical trial sites, including any government-imposed travel restrictions or quarantines that will impact the ability or willingness of patients, employees or contractors to travel to our clinical trial sites or secure visas or entry permissions, a loss of face-to-face meetings and other interactions with potential partners, any of which could delay or adversely impact the conduct or progress of our prospective clinical trials;

    the potential negative affect on the operations of our third-party manufacturers;

    interruption in global shipping affecting the transport of clinical trial materials, such as patient samples, conditioning drugs and other supplies used in our prospective clinical trials; and

    business disruptions caused by potential workplace, laboratory and office closures and an increased reliance on employees working from home, disruptions to or delays in ongoing laboratory experiments and operations, staffing shortages, travel limitations or mass transit disruptions, any of which could adversely impact our business operations or delay necessary interactions with local regulators, ethics committees and other important agencies and contractors.

        The COVID-19 pandemic has also impacted, and may continue to impact, our third-party suppliers and manufacturers, including through the effects of facility closures, reductions in operating hours, staggered shifts and other social distancing efforts, labor shortages, decreased productivity and unavailability of materials or components. While we maintain an inventory of materials used to conduct our research and development activities, a prolonged pandemic could lead to shortages in the raw materials necessary to manufacture our product candidates.

        We have taken temporary precautionary measures intended to help mitigate the risk of the virus to our employees, including temporarily requiring all office-based employees to work remotely, suspending all non-essential travel worldwide for our employees and discouraging employee attendance at industry events and in-person work-related meetings, which could negatively affect our business. In addition, we have limited laboratory capacity by initiating shiftwork within the laboratories to minimize number of staff in the facilities at any time. We have now moved from shiftwork but are not yet operating at full capacity in the laboratories. We cannot presently predict the scope and severity of the planned and potential shutdowns or disruptions of businesses and government agencies, such as the SEC or FDA.

        These and other factors arising from the coronavirus could worsen in countries that are already afflicted with the coronavirus or could continue to spread to additional countries. Any of these factors, and other factors related to any such disruptions that are unforeseen, could have a material adverse effect on our business, financial condition, results of operation or prospects. Further, uncertainty around these and related issues could lead to adverse effects on the economy of the United States and other economies, which could impact our ability to raise the necessary capital needed to develop and commercialize SIG-001 or any other product candidates.

Risks Related to This Offering and Ownership of Our Common Stock

We do not know whether a market will develop for our common stock or what the market price of our common stock will be, and, as a result, it may be difficult for you to sell your shares of our common stock.

        Before this offering, there was no public trading market for our common stock. If a market for our common stock does not develop or is not sustained, it may be difficult for you to sell your shares of

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common stock at an attractive price or at all. We cannot predict the prices at which our common stock will trade. It is possible that in one or more future periods our results of operations may be below the expectations of public market analysts and investors, and, as a result of these and other factors, the price of our common stock may fall.

You will incur immediate and substantial dilution as a result of this offering.

        If you purchase common stock in this offering, you will incur immediate and substantial dilution of $13.31 per share, representing the difference between the initial public offering price of $18.00 per share, and our pro forma as adjusted net tangible book value per share after giving effect to this offering and the automatic conversion of all outstanding shares of our preferred stock into common stock and the outstanding warrants to purchase shares of convertible preferred stock becoming warrants to purchase shares of common stock upon the closing of this offering. Moreover, we issued options in the past that allow the holders to acquire common stock at prices significantly below the initial public offering price. As of September 30, 2020, there were 4,109,728 shares subject to outstanding options with a weighted-average exercise price of $4.30 per share. To the extent that these outstanding options are ultimately exercised or the underwriters exercise their option to purchase additional shares, you will incur further dilution. For a further description of the dilution you will experience immediately after this offering, see "Dilution."

The market price of our common stock may be volatile, which could result in substantial losses for investors purchasing shares in this offering.

        The initial public offering price for our common stock was determined through negotiations with the underwriters. This initial public offering price may vary from the market price of our common stock after the offering. As a result, you may not be able to sell your common stock at or above the initial public offering price. Some of the factors that may cause the market price of our common stock to fluctuate include:

    the success of existing or new competitive product candidates or technologies;

    the timing and results of preclinical studies for any product candidates that we may develop;

    failure or discontinuation of any of our product development and research programs;

    results of preclinical studies, clinical trials, or regulatory approvals of product candidates of our competitors, or announcements about new research programs or product candidates of our competitors;

    developments or changing views regarding the use of engineered cell therapy and encapsulated cell therapy;

    commencement or termination of collaborations for our product development and research programs;

    regulatory or legal developments in the United States and other countries;

    developments or disputes concerning patent applications, issued patents, or other proprietary rights;

    the recruitment or departure of key personnel;

    the level of expenses related to any of our research programs, clinical development programs, or product candidates that we may develop;

    the results of our efforts to develop additional product candidates or products;

    actual or anticipated changes in estimates as to financial results, development timelines, or recommendations by securities analysts;

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    announcement or expectation of additional financing efforts;

    sales of our common stock by us, our insiders or other stockholders;

    expiration of market stand-off or lock-up agreement;

    variations in our financial results or those of companies that are perceived to be similar to us;

    changes in estimates or recommendations by securities analysts, if any, that cover our stock;

    changes in the structure of healthcare payment systems;

    market conditions in the pharmaceutical and biotechnology sectors;

    general economic, industry, and market conditions; and

    the other factors described in this "Risk Factors" section.

        In recent years, the stock market in general, and the market for pharmaceutical and biotechnology companies in particular, has experienced extreme price and volume fluctuations that have often been unrelated or disproportionate to changes in the operating performance of the companies whose stock is experiencing those price and volume fluctuations. Broad market and industry factors may seriously affect the market price of our common stock, regardless of our actual operating performance. These fluctuations may be even more pronounced in the trading market for our stock shortly following this offering. Following periods of such volatility in the market price of a company's securities, securities class action litigation has often been brought against that company. Because of the potential volatility of our stock price, we may become the target of securities litigation in the future. Securities litigation could result in substantial costs and divert management's attention and resources from our business.

If securities analysts do not publish research or reports about our business or if they publish negative evaluations of our stock, the price of our stock could decline.

        The trading market for our common stock will rely in part on the research and reports that industry or financial analysts publish about us or our business. We do not currently have and may never obtain research coverage by industry or financial analysts. If no or few analysts commence coverage of us, the trading price of our stock would likely decrease. Even if we do obtain analyst coverage, if one or more of the analysts covering our business downgrade their evaluations of our stock, the price of our stock could decline. If one or more of these analysts cease to cover our stock, we could lose visibility in the market for our stock, which in turn could cause our stock price to decline.

A significant portion of our total outstanding shares is restricted from immediate resale but may be sold into the market in the near future, which could cause the market price of our common stock to decline significantly, even if our business is doing well.

        Sales of a substantial number of shares of our common stock in the public market could occur at any time. These sales, or the perception in the market that the holders of a large number of shares of common stock intend to sell shares, could reduce the market price of our common stock. After this offering and after giving effect to the conversion of all outstanding shares of our preferred stock into shares of our common stock upon the closing of this offering, we will have 30,190,271 shares of common stock outstanding, or 31,240,271 shares if the underwriters exercise their option to purchase additional shares in full, in each case based on the shares of our common stock outstanding as of October 31, 2020. Of these shares, the 7,000,000 shares (or 8,050,000 shares if the underwriters exercise their option to purchase additional shares in full) we are selling in this offering may be resold in the public market immediately, unless purchased by our affiliates. The remaining 23,190,271 shares are currently restricted under securities laws or as a result of lock-up or other agreements, but will be able to be sold after this offering as described in the "Shares Eligible for Future Sale" section of this prospectus. Moreover, after this offering,

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holders of an aggregate of 10,414,815 shares of our common stock (including shares of our common stock issuable upon the conversion of all outstanding shares of our convertible preferred stock, including shares of our Series B-1 convertible preferred stock) will have rights, subject to conditions, to require us to file registration statements covering their shares or to include their shares in registration statements that we may file for ourselves or other stockholders. We also plan to register all shares of common stock that we may issue under our equity compensation plans or that are issuable upon exercise of outstanding options. Once we register these shares, they can be freely sold in the public market upon issuance and once vested, subject to volume limitations applicable to affiliates and the lock-up agreements described in the "Underwriting" section of this prospectus. If any of these additional shares are sold, or if it is perceived that they will be sold, in the public market, the market price of our common stock could decline.

Insiders will continue to have substantial influence over us after this offering, which could limit your ability to affect the outcome of key transactions, including a change of control.

        After this offering, our directors and executive officers and their affiliates will beneficially own shares representing approximately 52.4% of our outstanding common stock. As a result, these stockholders, if they act together, will be able to influence our management and affairs and all matters requiring stockholder approval, including the election of directors and approval of significant corporate transactions. This concentration of ownership may have the effect of delaying or preventing a change in control of our company and might affect the market price of our common stock.

If we fail to establish and maintain proper and effective internal control over financial reporting, our operating results and our ability to operate our business could be harmed.

        Ensuring that we have adequate internal financial and accounting controls and procedures in place so that we can produce accurate financial statements on a timely basis is a costly and time-consuming effort that needs to be re-evaluated frequently. Our internal control over financial reporting is a process designed to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements in accordance with generally accepted accounting principles. In connection with this offering, we intend to begin the process of documenting, reviewing and improving our internal controls and procedures for compliance with Section 404 of the Sarbanes-Oxley Act of 2002, or SOX, which will require annual management assessment of the effectiveness of our internal control over financial reporting.

        Implementing any appropriate changes to our internal controls may distract our officers and employees, entail substantial costs to modify our existing processes and take significant time to complete. These changes may not, however, be effective in maintaining the adequacy of our internal controls, and any failure to maintain that adequacy, or consequent inability to produce accurate financial statements on a timely basis, could increase our operating costs and harm our business. In addition, investors' perceptions that our internal controls are inadequate or that we are unable to produce accurate financial statements on a timely basis may harm our common stock price and make it more difficult for us to effectively market and sell our product candidates to customers.

We are an "emerging growth company" and a "smaller reporting company," and the reduced disclosure requirements applicable to emerging growth companies may make our common stock less attractive to investors.

        We are an "emerging growth company," as defined in the Jumpstart Our Business Startups Act of 2012, or the JOBS Act, and may remain an emerging growth company for up to five years. For so long as we remain an emerging growth company, we are permitted and plan to rely on exemptions from certain disclosure requirements that are applicable to other public companies that are not emerging growth companies. These exemptions include not being required to comply with the auditor attestation requirements of SOX Section 404, not being required to comply with any requirement that may be adopted by the Public Company Accounting Oversight Board regarding mandatory audit firm rotation or a supplement to the auditor's report providing additional information about the audit and the financial

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statements, reduced disclosure obligations regarding executive compensation, and exemptions from the requirements of holding a nonbinding advisory vote on executive compensation and stockholder approval of any golden parachute payments not previously approved. As a result, the information we provide stockholders will be different than the information that is available with respect to other public companies. In this prospectus, we have not included all of the executive compensation related information that would be required if we were not an emerging growth company and we have provided only two years of audited financial statements and only two years of related "Management's Discussion and Analysis of Financial Condition and Results of Operations." If some investors find our common stock less attractive as a result, there may be a less active trading market for our common stock, and our stock price may be more volatile.

        In addition, the JOBS Act provides that an emerging growth company can take advantage of an extended transition period for complying with new or revised accounting standards. This allows an emerging growth company to delay the adoption of certain accounting standards until those standards would otherwise apply to private companies.

        Further, even after we no longer qualify as an emerging growth company, we may still qualify as a "smaller reporting company," which would allow us to take advantage of many of the same exemptions from disclosure requirements, including reduced disclosure obligations regarding executive compensation in our periodic reports and proxy statements. In addition, if we are a smaller reporting company, we would not be required to comply with the auditor attestation requirements of Section 404 of the Sarbanes-Oxley Act, or Section 404. These exemptions and reduced disclosures in our SEC filings due to our status as a smaller reporting company may make it harder for investors to analyze our results of operations and financial prospects. We cannot predict if investors will find our common stock less attractive because we may rely on these exemptions. If some investors find our common stock less attractive as a result, there may be a less active trading market for our common stock and our stock prices may be more volatile.

We will incur increased costs as a result of operating as a public company, and our management will be required to devote substantial time to new compliance initiatives and corporate governance practices.

        As a public company, and particularly after we are no longer an "emerging growth company," we will incur significant legal, accounting, and other expenses that we did not incur as a private company. SOX, the Dodd-Frank Wall Street Reform and Consumer Protection Act, the listing requirements of The Nasdaq Global Select Market, and other applicable securities rules and regulations impose various requirements on public companies, including establishment and maintenance of effective disclosure and financial controls and corporate governance practices. We expect that we will need to hire additional accounting, finance, and other personnel in connection with our becoming, and our efforts to comply with the requirements of being, a public company, and our management and other personnel will need to devote a substantial amount of time towards maintaining compliance with these requirements. These requirements will increase our legal and financial compliance costs and will make some activities more time-consuming and costly. For example, we expect that the rules and regulations applicable to us as a public company may make it more difficult and more expensive for us to obtain director and officer liability insurance. We are currently evaluating these rules and regulations and cannot predict or estimate the amount of additional costs we may incur or the timing of such costs. These rules and regulations are often subject to varying interpretations, in many cases due to their lack of specificity, and, as a result, their application in practice may evolve over time as new guidance is provided by regulatory and governing bodies. This could result in continuing uncertainty regarding compliance matters and higher costs necessitated by ongoing revisions to disclosure and governance practices.

        Pursuant to SOX Section 404, we will be required to furnish a report by our management on our internal control over financial reporting beginning with our second filing of an Annual Report on Form 10-K with the SEC after we become a public company. However, while we remain an emerging growth company and smaller reporting company, we will not be required to include an attestation report on internal control over financial reporting issued by our independent registered public accounting firm.

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To achieve compliance with SOX Section 404 within the prescribed period, we will be engaged in a process to document and evaluate our internal control over financial reporting, which is both costly and challenging. In this regard, we will need to continue to dedicate internal resources, potentially engage outside consultants, adopt a detailed work plan to assess and document the adequacy of internal control over financial reporting, continue steps to improve control processes as appropriate, validate through testing that controls are functioning as documented, and implement a continuous reporting and improvement process for internal control over financial reporting. Despite our efforts, there is a risk that we will not be able to conclude, within the prescribed timeframe or at all, that our internal control over financial reporting is effective as required by SOX Section 404. If we identify one or more material weaknesses, it could result in an adverse reaction in the financial markets due to a loss of confidence in the reliability of our financial statements.

We have broad discretion in the use of the net proceeds from this offering and may not use them effectively.

        We cannot specify with certainty the particular uses of the net proceeds we will receive from this offering. Our management will have broad discretion in the application of the net proceeds, including for any of the purposes described in "Use of Proceeds." Accordingly, you will have to rely upon the judgment of our management with respect to the use of the proceeds, with only limited information concerning management's specific intentions. Our management may spend a portion or all of the net proceeds from this offering in ways that our stockholders may not desire or that may not yield a favorable return. The failure by our management to apply these funds effectively could harm our business, financial condition, results of operations and prospects. Pending their use, we may invest the net proceeds from this offering in a manner that does not produce income or that loses value.

We do not expect to pay any dividends for the foreseeable future. Investors in this offering may never obtain a return on their investment.

        You should not rely on an investment in our common stock to provide dividend income. We do not anticipate that we will pay any dividends to holders of our common stock in the foreseeable future. Instead, we plan to retain any earnings to maintain and expand our existing operations. In addition, any future credit facility may contain terms prohibiting or limiting the amount of dividends that may be declared or paid on our common stock. Accordingly, investors must rely on sales of their common stock after price appreciation, which may never occur, as the only way to realize any return on their investment. As a result, investors seeking cash dividends should not purchase our common stock.

Unfavorable global economic conditions could adversely affect our business, financial condition or results of operations.

        Our results of operations could be adversely affected by general conditions in the global economy and in the global financial markets. For example, in 2008, the global financial crisis caused extreme volatility and disruptions in the capital and credit markets and the current COVID-19 pandemic has caused significant volatility and uncertainty in U.S. and international markets. See "A pandemic, epidemic, or outbreak of an infectious disease, such the COVID-19 pandemic, may materially and adversely affect our business and our financial results and could cause a disruption to the development or supply of SIG-001 or any other product candidates." A severe or prolonged economic downturn, or additional global financial crises, could result in a variety of risks to our business, including weakened demand for our product candidates, if approved, or our ability to raise additional capital when needed on acceptable terms, if at all. A weak or declining economy could also strain our suppliers, possibly resulting in supply disruption. Any of the foregoing could harm our business and we cannot anticipate all of the ways in which the current economic climate and financial market conditions could adversely impact our business.

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U.S. federal income tax reform could adversely affect our business and financial condition.

        The rules dealing with U.S. federal, state, and local income taxation are constantly under review by persons involved in the legislative process and by the Internal Revenue Service and the U.S. Treasury Department. Changes to tax laws (which changes may have retroactive application) could adversely affect us or holders of our common stock. In recent years, many such changes have been made and changes are likely to continue to occur in the future. For example, on March 27, 2020, President Trump signed into law the Coronavirus Aid, Relief, and Economic Security Act, or the CARES Act, which included certain changes in tax law intended to stimulate the U.S. economy in light of the COVID-19 coronavirus outbreak, including temporary beneficial changes to the treatment of net operating losses, interest deductibility limitations and payroll tax matters. Additionally, on December 22, 2017, President Trump signed into law the Tax Cuts and Jobs Act, or the TCJA, which significantly reformed the Internal Revenue Code of 1986, as amended, or the Code. The TCJA included significant changes to corporate and individual taxation, some of which could adversely impact an investment in our common stock. Future changes in tax laws could have a material adverse effect on our business, cash flow, financial condition or results of operations. We urge investors to consult with their legal and tax advisers regarding the implications of potential changes in tax laws on an investment in our common stock.

Our future ability to utilize our net operating loss carryforwards and certain other tax attributes may be limited.

        We have incurred substantial losses during our history, and we may never achieve profitability. To the extent that we continue to generate taxable losses, unused losses will carry forward to offset a portion of future taxable income, if any, subject to expiration of such carryforwards in the case of carryforwards generated prior to 2018. Additionally, we continue to generate business tax credits, including research and development tax credits, which generally may be carried forward to offset a portion of future taxable income, if any, subject to expiration of such credit carryforwards. Under Sections 382 and 383 of the Code, if a corporation undergoes an "ownership change," generally defined as a greater than 50 percentage point change (by value) in its equity ownership over a three-year period, the corporation's ability to use its pre-change net operating loss carryforwards, or NOLs, and other pre-change tax attributes (such as research and development tax credits) to offset its post-change income or taxes may be limited. Our prior equity offerings and other changes in our stock ownership have resulted in such ownership changes. We may experience additional ownership changes in the future as a result of this offering or subsequent shifts in our stock ownership, some of which are outside of our control. As a result, if we earn net taxable income, our ability to use our pre-change NOLs or other pre-change tax attributes to offset U.S. federal taxable income may be subject to limitations, which could potentially result in increased future tax liability to us. Additionally, for taxable years beginning after December 31, 2020, the deductibility of such U.S. federal net operating losses is limited to 80% of our taxable income in any future taxable year. There is a risk that due to changes under the TCJA, regulatory changes, or other unforeseen reasons, our existing NOLs or business tax credits could expire or otherwise be unavailable to offset future income tax liabilities. At the state level, there may also be periods during which the use of NOLs or business tax credits is suspended or otherwise limited, which could accelerate or permanently increase state taxes owed. For these reasons, we may not be able to realize a tax benefit from the use of our NOLs or tax credits, even if we attain profitability.

Provisions in our amended and restated certificate of incorporation, our amended and restated by-laws and Delaware law may have anti-takeover effects that could discourage an acquisition of us by others, even if an acquisition would be beneficial to our stockholders, and may prevent attempts by our stockholders to replace or remove our current management.

        Our amended and restated certificate of incorporation, amended and restated by-laws and Delaware law contain provisions that may have the effect of discouraging, delaying or preventing a change in control of us or changes in our management that stockholders may consider favorable, including transactions in

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which you might otherwise receive a premium for your shares. Our amended and restated certificate of incorporation and by-laws, which will become effective upon the closing of this offering, include provisions that:

    authorize "blank check" preferred stock, which could be issued by our board of directors without stockholder approval and may contain voting, liquidation, dividend and other rights superior to our common stock;

    create a classified board of directors whose members serve staggered three-year terms;

    specify that special meetings of our stockholders can be called only by our board of directors;

    prohibit stockholder action by written consent;

    establish an advance notice procedure for stockholder approvals to be brought before an annual meeting of our stockholders, including proposed nominations of persons for election to our board of directors;

    provide that vacancies on our board of directors may be filled only by a majority of directors then in office, even though less than a quorum;

    provide that our directors may be removed for cause only;

    specify that no stockholder is permitted to cumulate votes at any election of directors;

    expressly authorized our board of directors to modify, alter or repeal our amended and restated by-laws; and

    require supermajority votes of the holders of our common stock to amend specified provisions of our amended and restated certificate of incorporation and amended and restated by-laws.

        These provisions, alone or together, could delay or prevent hostile takeovers and changes in control or changes in our management. These provisions could also limit the price that investors might be willing to pay in the future for shares of our common stock, thereby depressing the market price of our common stock.

        In addition, because we are incorporated in the State of Delaware, we are governed by the provisions of Section 203 of the General Corporation Law of the State of Delaware, or the DGCL, which prohibits a person who owns in excess of 15% of our outstanding voting stock from merging or combining with us for a period of three years after the date of the transaction in which the person acquired in excess of 15% of our outstanding voting stock, unless the merger or combination is approved in a prescribed manner.

        Any provision of our amended and restated certificate of incorporation, amended and restated by-laws or Delaware law that has the effect of delaying or deterring a change in control could limit the opportunity for our stockholders to receive a premium for their shares of our common stock, and could also affect the price that some investors are willing to pay for our common stock.

Our amended and restated certificate of incorporation designates the state or federal courts within the State of Delaware as the exclusive forum for certain types of actions and proceedings that may be initiated by our stockholders, which could limit our stockholders' ability to obtain a favorable judicial forum for disputes with us or our directors, officers or employees.

        Our amended and restated certificate of incorporation provides that, subject to limited exceptions, the state or federal courts within the State of Delaware will be exclusive forums for (1) any derivative action or proceeding brought on our behalf, (2) any action asserting a claim of breach of a fiduciary duty owed by any of our directors, officers or other employees to us or our stockholders, (3) any action asserting a claim against us arising pursuant to any provision of the DGCL, our amended and restated certificate of incorporation or our amended and restated by-laws, (4) any action to interpret, apply, enforce or

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determine the validity of our amended and restated certificate of incorporation or our amended and restated bylaws or (5) any other action asserting a claim against us that is governed by the internal affairs doctrine; provided that, the exclusive forum provision will not apply to suits brought to enforce any liability or duty created by the Exchange Act or to any claim for which the federal courts have exclusive jurisdiction. Our amended and restated certificate of incorporation also provides that, unless we consent in writing to the selection of an alternative forum, the U.S. federal district courts shall be the exclusive forum for the resolution of any claims arising under the Securities Act. Any person or entity purchasing or otherwise acquiring any interest in shares of our capital stock shall be deemed to have notice of and to have consented to the provisions of our amended and restated certificate of incorporation described above. This choice of forum provision may limit a stockholder's ability to bring a claim in a judicial forum that it finds favorable for disputes with us or our directors, officers or other employees, which may discourage such lawsuits against us and our directors, officers and employees.

        Alternatively, if a court were to find these provisions of our amended and restated certificate of incorporation inapplicable to, or unenforceable in respect of, one or more of the specified types of actions or proceedings, we may incur additional costs associated with resolving such matters in other jurisdictions, which could adversely affect our business and financial condition.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

        This prospectus contains forward-looking statements that are based on management's beliefs and assumptions and on information currently available to management. All statements other than statements of historical facts contained in this prospectus are forward-looking statements. In some cases, you can identify forward-looking statements by terms such as "may," "will," "should," "expect," "plan," "anticipate," "could," "intend," "target," "project," "contemplate," "believe," "estimate," "predict," "potential" or "continue" or the negative of these terms or other similar expressions, although not all forward-looking statements contain these words. Forward-looking statements include, but are not limited to, statements concerning:

    the initiation, timing, progress and results of our research and development programs, preclinical studies and clinical trials, including the timing of our clinical trials for SIG-001 for the treatment of Hemophilia A and the submission of INDs or CTAs for our other product candidates;

    our ability to advance any product candidates that we may develop and successfully complete any clinical studies, including the manufacture of any such product candidates;

    our ability to leverage our initial programs to develop additional product candidates using the SLTx platform;

    the impact of the COVID-19 pandemic on our business operations, including our research and development programs, preclinical studies and clinical trials;

    developments related to our competitors and our industry;

    our ability to expand the target populations of our programs;

    our ability to identify and enter into future license agreements and collaborations;

    our ability to successfully scale our manufacturing capabilities;

    regulatory developments in the United States and foreign countries;

    our ability to attract and retain key scientific and management personnel;

    the scope of protection we are able to establish and maintain for intellectual property rights covering our product candidates and technologies; and

    our use of proceeds from this offering, estimates of our expenses, capital requirements and needs for additional financing.

        The forward-looking statements in this prospectus are only predictions and are based largely on our current expectations and projections about future events and financial trends that we believe may affect our business, financial condition and results of operations. These forward-looking statements speak only as of the date of this prospectus and are subject to a number of known and unknown risks, uncertainties and assumptions, including those described under the sections in this prospectus entitled "Risk Factors" and "Management's Discussion and Analysis of Financial Condition and Results of Operations" and elsewhere in this prospectus. Because forward-looking statements are inherently subject to risks and uncertainties, some of which cannot be predicted or quantified and some of which are beyond our control, you should not rely on these forward-looking statements as guarantees of future events. The events and circumstances reflected in our forward-looking statements may not be achieved or occur and actual results could differ materially from those projected in the forward-looking statements. Moreover, we operate in an evolving environment. New risks and uncertainties may emerge from time to time, and it is not possible for management to predict all risks and uncertainties. Except as required by applicable law, we are not obligated to publicly update or revise any forward-looking statements contained herein, whether as a result of any new information, future events, changed circumstances or otherwise.

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USE OF PROCEEDS

        We estimate that the net proceeds to us from the sale of the shares of our common stock in this offering will be approximately $114.0 million, or approximately $131.6 million if the underwriters exercise their option to purchase additional shares in full, based on the initial public offering price of $18.00 per share, and after deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us.

        The principal purposes of this offering are to increase our financial flexibility, create a public market for our common stock and to facilitate our access to the public equity markets.

        We intend to use the net proceeds from this offering, together with our cash on hand, as follows:

    approximately $30.0 million to $35.0 million to progress our Phase 1/2 clinical trial and ongoing development for SIG-001 for treatment of Hemophilia A;

    approximately $30.0 million to $35.0 million to continue to scale our GMP manufacturing processes for our lead candidates SIG-001 and SIG-005;

    approximately $25.0 million to $30.0 million for continued advancement of our platform technologies and discovery-stage research for other potential programs;

    approximately $10.0 million to $15.0 million for IND-enabling studies and the potential initiation of clinical studies for certain of our current programs; and

    the remainder for working capital and general corporate purposes.

        We may also use a portion of the net proceeds from this offering to acquire, in-license or invest in products, technologies or businesses that are complementary to our business. The amounts and timing of our actual expenditures will depend on numerous factors, including the progress of our preclinical development efforts, our operating costs and other factors described under "Risk Factors" in this prospectus.

        Our expected use of net proceeds from this offering represents our current intentions based upon our present plans and business condition. As of the date of this prospectus, we cannot predict with complete certainty all of the particular uses for the net proceeds to be received upon the completion of this offering or the actual amounts that we will spend on the uses set forth above. We expect the net proceeds from this offering, together with our existing cash, will not be sufficient for us to advance any of our programs through regulatory approval or complete our GMP manufacturing processes for our lead candidates SIG-001 and SIG-005. Although we do not currently know the extent of additional funds that will be required, we expect that we will need to raise additional capital to complete these efforts. We expect to seek additional financing through a combination of equity offerings, debt financings, collaborations and strategic alliances. See "Management's Discussion and Analysis of Financial Condition and Results of Operations—Liquidity and Capital Resources."

        Based on our planned use of the net proceeds from this offering and our existing cash, we estimate that such funds will be sufficient to enable us to fund our operating expenses, debt service payments and capital expenditure requirements into the fourth quarter of 2022. We have based this estimate on assumptions that may prove to be wrong, and we could use our available capital resources sooner than we currently expect.

        We may find it necessary or advisable to use the net proceeds for other purposes, and we will have broad discretion in the application of the net proceeds. Pending the uses described above, we plan to invest the net proceeds from this offering in short-term, interest-bearing obligations, investment-grade instruments, certificates of deposit or direct or guaranteed obligations of the U.S. government.

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DIVIDEND POLICY

        We have never declared or paid any cash dividends on our capital stock. We intend to retain future earnings, if any, to finance the operation and expansion of our business and do not anticipate paying any cash dividends in the foreseeable future. Any future determination related to our dividend policy will be made at the discretion of our board of directors after considering our financial condition, results of operations, capital requirements, business prospects and other factors our board of directors deems relevant, and subject to the restrictions contained in any future financing instruments. Our ability to pay cash dividends on our capital stock in the future may also be limited by the terms of any preferred securities we may issue or agreements governing any indebtedness we may incur.

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CAPITALIZATION

        The following table sets forth our cash and capitalization as of September 30, 2020:

    on an actual basis;

    on a pro forma basis, to give effect to (i) the issuance and sale of 3,550,000 shares of Series B-1 convertible preferred stock for proceeds of $24.9 million in October 2020, (ii) the automatic conversion of all outstanding shares of convertible preferred stock, which includes 3,550,000 shares of our Series B-1 convertible preferred stock issued in October 2020, into an aggregate of 17,727,101 shares of common stock upon the closing of this offering, (iii) all outstanding warrants to purchase shares of convertible preferred stock becoming warrants to purchase shares of common stock and (iv) the filing and effectiveness of our restated articles of organization; and

    on a pro forma as adjusted basis, to give further effect to our issuance and sale of 7,000,000 shares of our common stock in this offering at the initial public offering price of $18.00 per share, after deducting estimated underwriting discounts and commissions and estimated offering expenses.

        The pro forma as adjusted information below is illustrative only and our capitalization following the closing of this offering will change based on the initial public offering price and other terms of this offering determined at pricing. You should read the information in this table together with the financial statements and related notes as appearing at the end of this prospectus and the information set forth under the sections titled "Selected Financial Data" and "Management's Discussion and Analysis of Financial Condition and Results of Operations."

 
  As of September 30, 2020  
 
  Actual   Pro forma   Pro Forma
as adjusted
 
 
  (in thousands, except share and per share amounts)
 

Cash

  $ 62,642   $ 87,492   $ 201,617  

Preferred stock warrant liability

  $ 446   $   $  

Long-term debt, net of discount, including current portion

    19,740     19,740     19,740  

Convertible preferred stock (Series A, A-1, A-3 and B), $0.001 par value: 36,494,335 shares authorized, actual; 36,336,001 shares issued and outstanding, actual; no shares authorized, issued or outstanding, pro forma and pro forma as adjusted

    117,149          

Stockholders' equity (deficit):

                   

Preferred stock, $0.001 par value; no shares authorized, issued or outstanding, actual; 25,000,000 shares authorized and no shares issued or outstanding, pro forma and pro forma as adjusted

             

Common stock, $0.001 par value; 60,958,334 shares authorized, actual; 5,341,090 shares issued and outstanding, actual; 175,000,000 shares authorized, pro forma; 23,068,191 shares issued and outstanding, pro forma; 175,000,000 shares authorized, pro forma as adjusted; 30,190,271 shares issued and outstanding, pro forma as adjusted

    5     23     30  

Additional paid-in capital

    5,814     148,241     262,359  

Accumulated deficit

    (120,739 )   (120,739 )   (120,739 )

Total stockholders' equity (deficit)

    (114,920 )   27,525     141,650  

Total capitalization

  $ 22,415   $ 47,265   $ 161,390  

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        The outstanding share information in the table above as of September 30, 2020 excludes:

    70,367 shares of common stock issuable upon the exercise of outstanding warrants to purchase preferred stock as of September 30, 2020 that will become warrants to purchase shares of common stock at a weighted-average exercise price of $8.17 per share upon the closing of this offering;

    4,109,728 shares of common stock issuable upon the exercise of stock options outstanding as of September 30, 2020 under the 2016 Plan at a weighted average exercise price of $4.30 per share;

    349,172 additional shares of common stock available for future issuance as of September 30, 2020 under our 2016 Plan; and

    1,500,000 shares of common stock reserved for issuance under the 2020 Plan which will become effective in connection with this offering.

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DILUTION

        If you invest in our common stock in this offering, your ownership interest will be diluted immediately to the extent of the difference between the initial public offering price per share of our common stock and the pro forma as adjusted net tangible book value per share of our common stock after this offering.

        Our historical net tangible book value (deficit) as of September 30, 2020 was $(116.5) million, or $(21.82) per share of common stock. Our historical net tangible book value (deficit) is the amount of our total tangible assets less our total liabilities and the carrying value of our preferred stock, which is not included within stockholders' equity (deficit). Historical net tangible book value (deficit) per share represents historical net tangible book value (deficit) divided by the 5,341,090 shares of common stock outstanding as of September 30, 2020.

        Our pro forma net tangible book value as of September 30, 2020 was $25.9 million or $1.12 per share of common stock, based on the total number of shares of our common stock outstanding, pro forma, as of September 30, 2020. Pro forma net tangible book value represents the amount of our total tangible assets less our liabilities, after giving effect to (i) the issuance and sale of 3,550,000 shares of Series B-1 convertible preferred stock for proceeds of $24.9 million in October 2020, (ii) the automatic conversion of the outstanding convertible preferred stock, which includes 3,550,000 shares of our Series B-1 convertible preferred stock issued in October 2020, into an aggregate of 17,727,101 shares of common stock upon the closing of this offering and (iii) all outstanding warrants to purchase shares of convertible preferred stock becoming warrants to purchase shares of common stock.

        After giving effect to our issuance and sale of shares of common stock in this offering at the initial public offering price $18.00 per share, and after deducting the estimated underwriting discounts and commissions and estimated offering expenses payable by us, our pro forma as adjusted net tangible book value as of September 30, 2020 would have been $141.7 million, or $4.69 per share. This represents an immediate increase in pro forma as adjusted net tangible book value of $3.57 per share to existing stockholders and an immediate dilution of $13.31 in pro forma as adjusted net tangible book value per share to new investors participating in this offering. Dilution per share to new investors is determined by subtracting pro forma as adjusted net tangible book value per share after this offering from the initial public offering price per share paid by new investors. The following table illustrates this dilution on a per share basis:

Initial public offering price per share

        $ 18.00  

Historical net tangible book value per share of common stock as of September 30, 2020

  $ (21.82 )      

Increase per share in net tangible book value per share of common stock attributable to pro forma adjustments

    22.94        

Pro forma net tangible book value per share of common stock as of September 30, 2020

    1.12        

Increase in net tangible book value per share of common stock attributable to this offering

    3.57        

Pro forma as adjusted net tangible book value per share of common stock after this offering

          4.69  

Dilution per share of common stock to new investors participating in this offering

        $ 13.31  

        If the underwriters exercise in full their option to purchase additional shares of common stock from us in this offering, our pro forma as adjusted net tangible book value per share after the offering would be $5.10, representing an immediate increase in pro forma as adjusted net tangible book value per share of $0.41 to existing stockholders and immediate dilution in pro forma as adjusted net tangible book value per share of $0.41 to new investors purchasing common stock in this offering, based on the initial public

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offering price of $18.00 per share, and after deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us.

        The following table summarizes, as of September 30, 2020, on the pro forma as adjusted basis described above, the total number of shares of common stock purchased from us on an as converted to common stock basis, the total consideration and the average price per share (1) paid by existing stockholders and (2) to be paid by new investors participating in this offering at the initial public offering price of $18.00 per share, before deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us. As the table shows, new investors purchasing common stock in this offering will pay an average price per share substantially higher than our existing stockholders paid.

 
  Shares purchased   Total consideration    
 
 
  Average price
per share
 
 
  Number   Percent   Amount   Percent  

Existing stockholders

    23,190,271     76.8 % $ 142,886,658     53.1 % $ 6.16  

New investors

    7,000,000     23.2 %   126,000,000     46.9 %   18.00  

Total

    30,190,271     100.0 % $ 268,886,658     100.0 % $ 8.91  

        The table above assumes no exercise of the underwriters' option to purchase additional shares in this offering. If the underwriters' option to purchase additional shares is exercised in full, the number of shares of common stock held by existing stockholders would be reduced to 74.2% of the total number of shares of common stock to be outstanding upon completion of this offering, and the number of shares of common stock held by new investors participating in this offering will be further increased to 25.8% of the total number of shares of our common stock to be outstanding upon completion of the offering.

        The share information presented in the tables and discussions above as of September 30, 2020 excludes:

    70,367 shares of common stock issuable upon the exercise of outstanding warrants to purchase preferred stock as of September 30, 2020 at a weighted-average exercise price of $8.17 per share that will become warrants to purchase shares of common stock upon the closing of this offering;

    4,109,728 shares of common stock issuable upon the exercise of stock options outstanding as of September 30, 2020 under the 2016 Plan at a weighted average exercise price of $4.30 per share;

    349,172 additional shares of common stock available for future issuance as of September 30, 2020 under our 2016 Plan; and

    1,500,000 shares of common stock reserved for issuance under the 2020 Plan which will become effective in connection with this offering.

        New investors will experience further dilution if any of our outstanding options and warrants are exercised, new options or warrants are issued and exercised under our equity incentive plans or we issue additional shares of common stock, other equity securities or convertible debt securities for lower consideration per share than in this offering in the future. In addition, we may choose to raise additional capital through the sale of equity or convertible debt securities due to market conditions or strategic considerations, even if we believe we have sufficient funds for our current or future operating plans. If we raise additional capital through the sale of equity or convertible debt securities, the issuance of these securities could result in further dilution to our stockholders.

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SELECTED FINANCIAL DATA

        You should read the following selected financial data together with the section titled "Management's Discussion and Analysis of Financial Condition and Results of Operations" of this prospectus and our financial statements and the related notes included elsewhere in this prospectus. The statements of operations and comprehensive loss data for the years ended December 31, 2018 and 2019 and the balance sheet data as of December 31, 2018 and 2019 have been derived from our audited financial statements included elsewhere in this prospectus. The statements of operations and comprehensive loss data for the nine months ended September 30, 2019 and 2020 and our balance sheet as of September 30, 2020 have been derived from our unaudited financial statements included elsewhere in this prospectus. In the opinion of management, the unaudited financial data reflect all adjustments, consisting only of normal recurring adjustments, necessary for a fair statement of such financial information in those statements. Our historical results are not necessarily indicative of the results that may be expected in the future.

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  Year Ended December 31,   Nine Months Ended
September 30,
 
 
  2018   2019   2019   2020  
 
  (in thousands, except share and per share amounts)
 

Statement of Operations and Comprehensive Loss Data:

                         

Revenue:

                         

Collaboration revenue from related party

  $ 4,637   $ 14,155   $ 11,057   $ 9,618  

Operating expenses:

                         

Research and development (inclusive of related party payments to MIT of $2,250, $411, $286 and $0 for the years ended December 31, 2018 and 2019 and the nine months ended September 30, 2019 and 2020, respectively)

    21,039     48,108     33,094     39,151  

General and administrative

    6,673     10,170     7,270     9,023  

Total operating expenses

    27,712     58,278     40,364     48,174  

Loss from operations

    (23,075 )   (44,123 )   (29,307 )   (38,556 )

Other income (expense):

                         

Interest income

    698     1,058     781     268  

Interest expense

    (289 )   (650 )   (462 )   (697 )

Other expense

    (81 )   (6 )   (6 )   (47 )

Change in fair value of preferred stock warrant liability

    (18 )   (204 )   (202 )   (44 )

Loss on extinguishment of debt

                    (343 )

Total other income (expense), net

    310     198     111     (863 )

Net loss and comprehensive loss

  $ (22,765 ) $ (43,925 ) $ (29,196 ) $ (39,419 )

Accretion of beneficial conversion feature

    (1,292 )            

Accretion of Series A convertible preferred stock to redemption value

    (1,698 )       (1,668 )    

Net loss attributable to common stockholders

  $ (25,755 ) $ (43,925 ) $ (30,864 ) $ (39,419 )

Net loss per share attributable to common stockholders—basic and diluted(1)

  $ (9.19 ) $ (10.74 ) $ (7.90 ) $ (7.56 )

Weighted average common stock outstanding—basic and diluted(1)

    2,802,634     4,090,691     3,904,805     5,214,818  

Pro forma net loss per share attributable to common stockholders—basic and diluted (unaudited)(1)

        $ (2.78 )       $ (1.87 )

Pro forma weighted average common stock outstanding—basic and diluted (unaudited)(1)

          15,703,464           21,042,984  

(1)
See Note 15 to our financial statements appearing at the end of this prospectus for details on the calculation of basic and diluted net loss per share attributable to common stockholders and unaudited pro forma basic and diluted net loss per share attributable to common stockholders.

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  December 31,    
 
 
  September 30,
2020
 
 
  2018   2019  
 
  (in thousands)
 

Balance Sheet Data:

                   

Cash

  $ 64,100   $ 76,069   $ 62,642  

Working capital(1)

    35,328     36,562     18,372  

Total assets

    67,665     90,378     77,453  

Long-term debt, net of discount, including current portion

    4,955     14,868     19,740  

Preferred stock warrant liability

    24     333     446  

Convertible preferred stock

    37,070     90,206     117,149  

Total stockholders' equity (deficit)

    (36,096 )   (77,762 )   (114,920 )

(1)
We define working capital as current assets less current liabilities.

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MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS OF OPERATIONS

        You should read the following discussion and analysis of our financial condition and results of operations together with the "Selected Financial Data" section of this prospectus and our financial statements and related notes included elsewhere in this prospectus. Some of the information contained in this discussion and analysis or set forth elsewhere in this prospectus, including information with respect to our plans and strategy for our business and related financing, includes forward-looking statements that involve risks and uncertainties. As a result of many factors, including those factors set forth in the "Risk Factors" section of this prospectus, our actual results could differ materially from the results described in or implied by the forward-looking statements contained in the following discussion and analysis.

Overview

        We are a clinical-stage biotechnology company pioneering a new class of therapeutics and seeking to develop functional cures for patients with chronic diseases by providing stable and durable levels of therapeutic molecules to patients. We have developed our Shielded Living Therapeutics, or SLTx, platform, which combines advanced cell engineering with cutting-edge innovations in biocompatible materials and enables our product candidates to produce a wide range of therapeutic molecules that may be missing or deficient, such as proteins, antibodies and hormones. We are designing our product candidates to be off-the-shelf, durable, controllable and redosable, without requiring modification of the patient's genes or partial or complete suppression of the patient's immune response, or immunosuppression. Our lead product candidate, SIG-001, is designed to prevent bleeding episodes in patients with moderate-severe to severe Hemophilia A by continuously secreting human Factor VIII, or hFVIII. We received acceptance of our IND submission in the United States and our CTA in the United Kingdom. We have initiated our Phase 1/2 clinical study of SIG-001 in Hemophilia A, with the first patient dosed in October 2020. Leveraging the modularity of our platform and our scientific and preclinical work to date, we are also advancing programs in additional rare blood disorders, lysosomal storage diseases, endocrine and other chronic disorders.

        Since our inception, we have devoted substantially all of our efforts to raising capital, obtaining financing, filing and prosecuting patent applications, organizing and staffing our company and incurring research and development costs related to advancing our biomedical platform. We do not have any products approved for sale and have not generated any revenue from product sales. To date, we have funded our operations primarily with proceeds from sales of convertible preferred stock, payments received under our collaboration agreement with Lilly and proceeds from borrowings under our credit facilities. Through September 30, 2020, we have received gross proceeds of $117.5 million from sales of our convertible preferred stock and net proceeds of $20.0 million through borrowings under our loan and security agreements with Pacific Western Bank, as amended and restated, or the 2019 Credit Facility and Oxford Finance LLC, or the 2020 Credit Facility. We have also partnered one of our encapsulation technology programs with Lilly. Under the terms of the partnership, we received an upfront payment of $62.5 million and we are eligible to receive additional milestone payments of up to $165.0 million upon achievement of certain regulatory milestones and sales-based milestones of up to $250.0 million for SIG-002. We are also eligible to receive tiered royalty payments in the mid-single digit to low-double digit percentages based on certain sales thresholds. Finally, Lilly is obligated to reimburse us for costs incurred to perform the research and development activities for the first developed product candidate that exceed $47.5 million.

        We have incurred significant operating losses since our inception. Our ability to generate any product revenue sufficient to achieve profitability will depend on the successful development and eventual commercialization of one or more of our product candidates. We reported net losses of $22.8 million and $43.9 million for the years ended December 31, 2018 and 2019, respectively, and $29.2 million and $39.4 million for the nine months ended September 30, 2019 and 2020, respectively. As of September 30,

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2020, we had an accumulated deficit of $120.7 million. We expect to continue to incur significant expenses and increasing operating losses for the foreseeable future. We expect that our expenses and capital expenditures will increase substantially in connection with our ongoing activities, particularly if and as we:

    initiate clinical trials for SIG-001 or any other product candidates;

    continue our current research programs and our preclinical development of product candidates from our current research programs;

    seek to identify additional research programs and additional product candidates;

    comply with regulatory requirements established by the FDA or EMA and other comparable foreign regulatory authorities;

    conduct preclinical studies for our product candidates;

    maintain, expand, enforce, defend and protect our intellectual property portfolio and provide reimbursement of third-party expenses related to our patent portfolio;

    seek marketing approvals for any of our product candidates that successfully complete clinical trials;

    ultimately establish a sales, marketing, and distribution infrastructure to commercialize any medicines for which we may obtain marketing approval;

    further develop our SLTx platform;

    hire additional research, development and manufacturing personnel;

    continue to hire clinical and hire commercial personnel;

    add operational, financial, corporate development and management information systems and legal personnel, including personnel to support our product development;

    acquire or in-license product candidates, intellectual property and technologies;

    develop an automated encapsulation system for future commercial scale manufacturing our SLTx platform or otherwise build or expand our manufacturing capabilities or capacity, including future manufacturing facilities;

    file, prosecute, defend, and enforce our patent claims and other intellectual property rights, including patent infringement actions brought by third parties against us regarding our investigational medicines or actions by us challenging the patent or intellectual property rights of others; and

    operate as a public company.

        We will not generate revenue from product sales unless and until we successfully complete clinical development and obtain regulatory approval for one or more of our product candidates. If we obtain regulatory approval for any of our product candidates, we expect to incur significant expenses related to developing our commercialization capability to support product sales, marketing and distribution. Further, following the completion of this offering, we expect to incur additional costs associated with operating as a public company.

        As a result, we will need substantial additional funding to support our continuing operations and pursue our growth strategy. Until such time as we can generate significant revenue from product sales, if ever, we expect to finance our operations through a combination of equity offerings, debt financings, or other capital sources, including collaborations with other companies and other strategic transactions. We may be unable to raise additional funds or enter into such other agreements or arrangements when needed on favorable terms, or at all. If we fail to raise capital or enter into such agreements as, and when, needed,

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we may have to significantly delay, scale back or discontinue the development and commercialization of one or more of our product candidates.

        Because of the numerous risks and uncertainties associated with pharmaceutical product development, we are unable to accurately predict the timing or amount of increased expenses or when, or if, we will be able to achieve or maintain profitability. Even if we are able to generate product sales, we may not become profitable. If we fail to become profitable or are unable to sustain profitability on a continuing basis, then we may be unable to continue our operations at planned levels and be forced to reduce or terminate our operations.

        We believe that the expected net proceeds from this offering, together with our existing cash will enable us to fund our operating expenses and capital expenditures into the fourth quarter of 2022. We have based this estimate on assumptions that may prove to be wrong, and we could exhaust our available capital resources sooner than we expect. See "—Liquidity and capital resources" and "Risk Factors—Risks Related to Our Financial Position and Need for Additional Capital."

        Without giving effect to the anticipated net proceeds from this offering, we expect that our existing cash as of September 30, 2020, together with proceeds of $24.9 million from the issuance of Series B-1 convertible preferred stock in October 2020, will be sufficient to fund our operating expenses and capital expenditures into the third quarter of 2021. Beyond that point, we will need to raise additional capital to finance our operations, which cannot be assured. We have concluded that this circumstance raises substantial doubt about our ability to continue as a going concern within one year after the August 21, 2020 issuance date of our financial statements for the year ended December 31, 2019 and the October 30, 2020 issuance date of our financial statements for the nine months ended September 30, 2020. See Note 1 to our financial statements appearing at the end of this prospectus for additional information on our assessment.

Impact of COVID-19

        In December 2019, a novel strain of coronavirus, which causes the disease known as COVID-19, was reported to have surfaced in Wuhan, China. Since then, COVID-19 coronavirus has spread globally. In March 2020, the World Health Organization declared the COVID-19 outbreak a pandemic and the U.S. government imposed travel restrictions on travel between the United States, Europe and certain other countries. The COVID-19 pandemic has impacted and may continue to impact the clinical sites and startup activities for our Phase 1/2 clinical trial, including third-party manufacturing and logistics providers, which would disrupt our clinical supply chain or the availability or cost of materials, and it may affect our ability to timely complete our clinical trials and delay the initiation and/or enrollment of any future clinical trials, disrupt regulatory activities or have other adverse effects on our business and operations. Within our laboratory facilities, we made several changes to our operations in response to COVID-19 in the first half of 2020. This response included reducing our capacity in the labs, based on a rolling prioritization of critical experiments and other work, and performing all office-based work outside of the office. To help reduce the risk to our employees, we took other precautionary measures with respect to lab-based activities, including:

    shift-based teams;

    mandatory wearing of masks and other personal protective equipment, or PPE; and

    deep cleaning of office, social distancing, occupancy limits, safety health checks and other preventive measures.

        In the second quarter, we initiated a phased reopening of our operations, restoring our lab-based capacity to 100%. We plan to continue many of the protective measures and are assessing when and how to resume normal operations for office based personnel. We are monitoring the potential impact of the COVID-19 pandemic on our business and financial statements. To date, we have not incurred impairment losses in the carrying values of our assets as a result of the pandemic and we are not aware of any specific

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related event or circumstance that would require us to revise our estimates reflected in our financial statements. We are also monitoring the impact of COVID-19. The effects of the public health directives and our work-from-home policies may negatively impact productivity, disrupt our business and delay clinical programs and timelines and future clinical trials, the magnitude of which will depend, in part, on the length and severity of the restrictions and other limitations on our ability to conduct business in the ordinary course. These and similar, and perhaps more severe, disruptions in our operations could negatively impact business, results of operations and financial condition, including our ability to obtain financing.

        We cannot be certain what the overall impact of the COVID-19 pandemic will be on our business and prospects. The extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations, financial condition and liquidity, including planned and future clinical trials and research and development costs, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19, the actions taken to contain or treat it, and the duration and intensity of the related effects.

Components of Results of Operations

Revenue

        To date, we have not generated any revenue from product sales and do not expect to generate any revenue from the sale of products in the near future. Substantially all of our revenue to date has been derived from the collaboration agreement with Lilly, which we entered into in 2018.

        If our development efforts for our product candidates are successful and result in regulatory approval or if we enter into license or collaboration agreements with third parties, we may generate revenue in the future from product sales, payments from license or collaboration agreements that we may enter into with third parties, or any combination thereof. We expect that our revenue for the next several years will be derived primarily from our collaboration agreement with Lilly as well as any additional collaborations that we may enter into in the future. We cannot provide assurance as to the timing of future milestone or royalty payments or that we will receive any of these payments at all.

Collaboration Revenue

        In April 2018, we entered into a License and Collaboration Agreement with Lilly, or the 2018 Lilly Agreement. Under the 2018 Lilly Agreement, we granted Lilly an exclusive worldwide, royalty-bearing license, including the right to grant sublicenses, to our encapsulation technology applied to islet cells. We are responsible our own costs and expenses associated with pre-clinical development of a product candidate, and completion of the studies and other criteria required for filing the first IND, up to $47.5 million. Lilly is responsible for filing the first IND, all subsequent clinical development and commercialization, all research, development and commercialization for any subsequent product candidates, as well as reimbursing us for research and development costs required for filing the first IND related to the first developed product candidate that exceed $47.5 million.

        We evaluated the 2018 Lilly Agreement under ASC 606 and concluded at the outset that there were two performance obligations under the arrangement: (1) exclusive license to research, develop, manufacture and commercialize licensed products, initial technology transfer, research activities (including pre-IND supply), cell line development and supply and product trademark election, or the Combined Performance Obligation; and (2) requirement to supply Lilly with the licensed product related to Phase I clinical trial, or Phase I Supply. We determined that the $62.5 million upfront payment represents the entirety of the consideration to be included in the transaction price as of the outset of the arrangement. We allocated $56.6 million of the transaction price to the Combined Performance Obligation and $5.9 million of the transaction price to the Phase 1 Supply at the outset of the arrangement. We recognize revenue for the Combined Performance Obligation as the research and development services are provided using an

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input method, based on the cumulative costs incurred compared to the total estimated costs expected to be incurred to satisfy the Combined Performance Obligation. The transfer of control to the customer occurs over the time period that the research and development services are to be provided by us, and this cost-to-cost method is, in management's judgment, the best measure of progress toward satisfying this performance obligation. We have determined that the Phase 1 Supply will be satisfied at a point in time when the customer obtains control of each unit of product. Therefore, we will recognize revenue as shipments of the Phase 1 Supply are made to Lilly.

        We reevaluate the transaction price and our total estimated costs expected to be incurred at the end of each reporting period and as uncertain events, such as changes to the expected timing and cost of certain research, development and manufacturing activities that we are responsible for, are resolved or other changes in circumstances occur, and, if necessary, we will adjust our estimate of the transaction price or our total estimated costs expected to be incurred.

        For further information on our revenue recognition policies, see "—Critical Accounting Policies and Significant Judgements and Estimates—Revenue Recognition"

Operating Expenses

Research and Development Expenses

        Research and development expenses consist primarily of costs incurred for our research activities, including our drug discovery efforts and the development of our platform and product candidates. We expense research and development costs as incurred, which include:

    employee-related expenses, including salaries, bonuses, benefits, stock-based compensation, other related costs for those employees involved in research and development efforts;

    expenses incurred in connection with the preclinical development of our product candidates and research programs, including under agreements with third parties, such as consultants, contractors, and CROs;

    the cost of raw materials and developing and scaling our manufacturing process and manufacturing product candidates for use in our research and preclinical studies, including under agreements with third parties, such as consultants, contractors, and CMOs;

    laboratory supplies and research materials;

    facilities, depreciation, and other expenses, which include direct and allocated expenses for rent and maintenance of facilities and insurance; and

    payments made under third-party licensing agreements.

        We expense research and development costs as incurred. Non-refundable advance payments that we make for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed, or when it is no longer expected that the goods will be delivered or the services rendered. Upfront payments under license agreements are expensed upon receipt of the license, and annual maintenance fees under license agreements are expensed in the period in which they are incurred. Milestone payments under license agreements are accrued, with a corresponding expense being recognized, in the period in which the milestone is determined to be probable of achievement and the related amount is reasonably estimable.

        Our direct external research and development expenses are tracked on a program-by-program basis, including our early-stage programs, and consist of costs that include fees, reimbursed materials, and other costs paid to consultants, contractors, contract manufacturing organizations or CMOs, and contract research organizations or CROs, in connection with our preclinical and manufacturing activities. Except

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for personnel expenses related to SIG-002, we do not allocate employee costs, costs associated with our discovery efforts, laboratory supplies and facilities expenses, including depreciation or other indirect costs, to specific product development programs because these costs are deployed across multiple programs and our platform and, as such, are not separately classified. The personnel expenses allocated to SIG-002 do not include stock-based compensation expense.

        Product candidates in later stages of clinical development generally have higher development costs than those in earlier stages of clinical development, primarily due to the increased size and duration of later stage clinical trials. We expect that our research and development expenses will increase substantially in connection with our planned preclinical and clinical development activities in the near term and in the future. At this time, we cannot accurately estimate or know the nature, timing and costs of the efforts that will be necessary to complete the preclinical and clinical development of any of our product candidates. The successful development and commercialization of our product candidates is highly uncertain. This is due to the numerous risks and uncertainties associated with product development and commercialization, including the following:

    the timing and progress of preclinical and clinical development activities;

    the number and scope of preclinical and clinical programs we decide to pursue;

    raising additional funds necessary to complete preclinical and clinical development of and commercialize our product candidates;

    the progress of the development efforts of parties with whom we may enter into collaboration arrangements;

    our ability to maintain our current research and development programs and to establish new ones;

    our ability to establish new licensing or collaboration arrangements;

    the successful initiation and completion of clinical trials with safety, tolerability and efficacy profiles that are satisfactory to the FDA, or any comparable foreign regulatory authority;

    the receipt and related terms of regulatory approvals from applicable regulatory authorities;

    the availability of raw materials for use in production of our product candidates;

    our ability to consistently manufacture our product candidates for use in clinical trials;

    our ability to establish and operate a manufacturing facility, or secure manufacturing supply through relationships with third parties;

    our ability to obtain and maintain patents, trade secret protection and regulatory exclusivity, both in the United States and internationally;

    our ability to protect our rights in our intellectual property portfolio;

    the commercialization of our product candidates, if and when approved;

    obtaining and maintaining third-party insurance coverage and adequate reimbursement;

    the acceptance of our product candidates, if approved, by patients, the medical community and third-party payors;

    competition with other products; and

    a continued acceptable safety profile of our therapies following approval.

        A change in the outcome of any of these variables with respect to the development of any of our product candidates could significantly change the costs and timing associated with the development of

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these product candidates. We may never succeed in obtaining regulatory approval for any of our product candidates.

General and Administrative Expenses

        General and administrative expenses consist primarily of salaries and personnel expenses, including stock-based compensation, for our personnel in executive, legal, finance and accounting, human resources, and other administrative functions. General and administrative expenses also include legal fees relating to patent and corporate matters; professional fees paid for accounting, auditing, consulting, and tax services; insurance costs; travel expenses; and facility costs not otherwise included in research and development expenses.

        We anticipate that our general and administrative expenses will increase in the future as we increase our headcount to support our continued research activities and development of our product candidates. We also anticipate that we will incur significantly increased accounting, audit, legal, regulatory, compliance and director and officer insurance costs as well as investor and public relations expenses associated with operating as a public company. Additionally, if and when we believe a regulatory approval of a product candidate appears likely, we anticipate an increase in payroll and other employee-related expenses as a result of our preparation for commercial operations, especially as it relates to the sales and marketing of that product candidate.

Other Income (Expense)

Change in Fair Value of Preferred Stock Warrant Liability

        In connection with our 2019 Credit Facility, we issued warrants to purchase Series A-1 and Series B convertible preferred stock. We classify these warrants as a liability on our balance sheet that we remeasure to fair value at each reporting date, and we recognize changes in the fair value of the warrant liability as a component of other income (expense) in our statements of operations and comprehensive loss. We will continue to recognize changes in the fair value of the warrant liability until the warrants are exercised, expire or qualify for equity classification. Upon the closing of this offering, the preferred stock warrants will become warrants to purchase common stock, and the fair value of the warrant liability at that time will be reclassified to additional paid-in capital.

Loss on Extinguishment of Debt

        In connection with the extinguishment of our 2019 Credit Facility a loss was recognized equal to the unamortized debt discount and extinguishment fees in the amount of $0.3 million.

Interest Income

        Interest income consists of interest earned on our invested cash balances. We expect our interest income will fluctuate based on the timing and ability to raise additional funds as well as the amount of expenditures for our platform development and ongoing business operations.

Interest Expense

        Interest expense consists of interest expense on outstanding borrowings under our loan and security agreements as well as amortization of debt discount and deferred financing costs.

Other Expense

        Other expense consists primarily of losses on the disposal of fixed assets.

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Income Taxes

        Since our inception, we have not recorded any income tax benefits for the net operating losses we have incurred in each year or for our earned research and development tax credits generated in each period, as we believe, based upon the weight of available evidence, that it is more likely than not that all of our net operating loss, or NOL, carryforwards and tax credit carryforwards will not be realized. As of December 31, 2019, we had U.S. federal NOL carryforwards of $31.0 million, which may be available to offset future taxable income, of which $10.0 million begin to expire in 2036 and of which $21.0 million do not expire but are (for taxable years beginning after December 31, 2020) generally limited in their usage to an annual deduction equal to 80% of annual taxable income. In addition, as of December 31, 2019, we had Massachusetts state net operating loss carryforwards of $29.4 million, which may be available to offset future taxable income and begin to expire at various times starting in 2037. We have recorded a full valuation allowance against our net deferred tax assets at each balance sheet date.

        On December 22, 2017, the Tax Cuts and Jobs Act, or the Tax Act, was signed into United States law. The Tax Act made broad and complex changes to existing tax law, which include, but are not limited to, (i) a reduction in the federal corporate income tax rate from a top marginal tax rate of 35% to a flat rate of 21%, effective as of January 1, 2018; (ii) the limitation of the deduction for NOLs to 80% of annual taxable income and elimination of NOL carrybacks, in each case, for losses arising in taxable years beginning after December 31, 2017 (though any such NOLs may be carried forward indefinitely); and (iii) the limitation of the deduction of certain executive compensation amounts. The reduction in U.S. federal corporate tax rate from 35% to 21% reduced the amounts of our gross deferred tax assets and our valuation allowance as of December 31, 2017. During the year ended December 31, 2018, we recorded no additional tax expense or benefit as a result of the Tax Act.

        On March 27, 2020, the Coronavirus Aid, Relief and Economic Security Act, or the CARES Act, was passed by the U.S. Congress and signed into United States law. The CARES Act, among other things, includes certain provisions for individuals and corporations (including a suspension on the application of the 80% limitation described above for taxable years beginning prior to January 1, 2021); however, these benefits did not impact our income tax provisions in the periods presented.

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Results of Operations

Comparison of the Nine Months Ended September 30, 2019 and 2020

        The following table summarizes our results of operations for the nine months ended September 30, 2019 and 2020:

 
  Nine Months Ended
September 30,
   
 
 
  2019   2020   Change  
 
  (in thousands)
   
 

Revenue

                   

Collaboration revenue from related party

  $ 11,057   $ 9,618   $ (1,439 )

Operating expenses:

                   

Research and development

    33,094     39,151     6,057  

General and administrative

    7,270     9,023     1,753  

Total operating expenses

    40,364     48,174     7,810  

Loss from operations

    (29,307 )   (38,556 )   (9,249 )

Other income (expense):

                   

Interest income

    781     268     (513 )

Interest expense

    (462 )   (697 )   (235 )

Other expense

    (6 )   (47 )   (41 )

Change in fair value of preferred stock warrant liability

    (202 )   (44 )   158  

Loss on Extinguishment of Debt

        (343 )   (343 )

Total other income (expense), net

    111     (863 )   (974 )

Net loss and comprehensive loss

  $ (29,196 ) $ (39,419 ) $ (10,223 )

Revenue

        Revenue was $11.1 million for the nine months ended September 30, 2019, compared to $9.6 million for the nine months ended September 30, 2020. The decrease in revenue of $1.4 million was due to a decrease in collaboration revenue from our collaboration agreement with Lilly, primarily related to the research and development activities performed under this agreement. In June 2020 and September 2020, revised estimates of total costs to complete the activities under the 2018 Lilly Agreement were presented to the JRC, which considered our experiences to date and the impact this has on our expected future research and development activities to satisfy the Combined Performance Obligation. This resulted in an increase to total estimated costs expected to be incurred of $23.0 million for the nine months ended September 30, 2020. This increase in total estimated costs impacted both our estimated transaction price for the 2018 Lilly Agreement, as Lilly is obligated to reimburse us if the costs exceed $47.5 million to complete the activities, and our input method used to recognize revenue, as this measure compares our cumulative costs incurred to our total estimated costs expected to be incurred. For the nine months ended September 30, 2020, the transaction price for the Combined Performance Obligation increased by $17.9 million based on the allocation of total transaction price to each performance obligation under the 2018 Lilly Agreement. Additionally, the transaction price for the Phase 1 Supply performance obligation increased by $1.9 million for the nine months ended September 30, 2020. However, revenue recognized for the nine months ended September 30, 2020, using the input measure, decreased as compared to revenue recognized for the nine months ended September 30, 2019 as the percentage of costs incurred to total costs expected to be incurred decreased as a result of the increased total estimated costs.

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Research and Development Expenses

        The following table summarizes our research and development expenses for the nine months ended September 30, 2019 and 2020:

 
  Nine Months Ended
September 30,
   
 
 
  2019   2020   Change  
 
  (in thousands)
   
 

Direct research and development expenses by program:

                   

SIG-001

  $ 5,252   $ 6,859   $ 1,607  

SIG-002

    6,982     9,666     2,684  

Platform and pipeline development

    11,456     11,469     13  

Unallocated expenses

                   

Personnel expenses (including stock-based compensation)

    7,511     9,249     1,738  

Facility related and other

    1,893     1,908     15  

Total research and development expenses

  $ 33,094   $ 39,151   $ 6,057  

        Research and development expenses were $33.1 million for the nine months ended September 30, 2019, compared to $39.2 million for the nine months ended September 30, 2020. The increase of $1.6 million related to program SIG-001 was due to the increase in external CRO and CMO fees related to the continued development of our Hemophilia A product candidates. The increase of $2.7 million related to program SIG-002 was due to the increase in personnel related costs, external research costs and related lab supplies needed to further develop our T1D program. There was no significant change related to platform and pipeline development expenses period to period. The increase of $1.7 million in unallocated personnel expenses was primarily due to increased headcount in our SIG-001 program and platform and pipeline development efforts. Personnel expenses included stock-based compensation expense of $0.5 million and $0.8 million for the nine months ended September 30, 2019 and 2020, respectively.

General and Administrative Expenses

        General and administrative expenses for the nine months ended September 30, 2019 were $7.3 million, compared to $9.0 million for the nine months ended September 30, 2020. This increase was driven by a $0.9 million increase in personnel expenses related to employee compensation, and a $0.8 million increase in other expenses primarily related to legal and professional costs. Personnel expenses included stock-based compensation expense of $0.9 million and $1.3 million for the nine months ended September 30, 2019 and 2020, respectively.

Other Income (Expense)

        Other income (expense) primarily consists of interest income (expense), the change in fair value of the preferred stock warrant liability and loss on extinguishment of debt. Other income (expense) for the nine months ended September 30, 2019 and 2020 was $0.1 million and $(0.9) million. The change was primarily due to the increase in interest expense on the outstanding borrowings under our 2019 Credit Facility and 2020 Credit Facility, a decrease in interest income on our invested cash balances, and the loss on extinguishment of our 2019 Credit Facility. The decrease in interest income was due to the decrease in average interest rates during the respective periods.

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Comparison of the Years Ended December 31, 2018 and 2019

        The following table summarizes our results of operations for the years ended December 31, 2018 and 2019:

 
  Year Ended
December 31,
   
 
 
  2018   2019   Change  
 
  (in thousands)
   
 

Revenue

                   

Collaboration revenue from related party

  $ 4,637   $ 14,155   $ 9,518  

Operating expenses:

                   

Research and development

    21,039     48,108     27,069  

General and administrative

    6,673     10,170     3,497  

Total operating expenses

    27,712     58,278     30,566  

Loss from operations

    (23,075 )   (44,123 )   (21,048 )

Other income (expense):

                   

Interest income

    698     1,058     360  

Interest expense

    (289 )   (650 )   (361 )

Other expense

    (81 )   (6 )   75  

Change in fair value of preferred stock warrant liability

    (18 )   (204 )   (186 )

Total other income (expense), net

    310     198     (112 )

Net loss and comprehensive loss

  $ (22,765 ) $ (43,925 ) $ (21,160 )

Revenue

        Revenue was $4.6 million for the year ended December 31, 2018, compared to $14.2 million for the year ended December 31, 2019. The increase in revenue of $9.5 million was due to an increase in collaboration revenue from the 2018 Lilly Agreement, primarily related to the research and development activities performed under this agreement. This increase was primarily due to a full year of revenue for the year ended December 31, 2019 compared to eight months of revenue for the year ended December 31, 2018.

Research and Development Expenses

        The following table summarizes our research and development expenses for the years ended December 31, 2018 and 2019:

 
  Year Ended
December 31,
   
 
 
  2018   2019   Change  
 
  (in thousands)
   
 

Direct research and development expenses by program:

                   

SIG-001

  $ 2   $ 10,195   $ 10,193  

SIG-002

    824     9,432     8,608  

Platform and pipeline development

    9,223     15,458     6,235  

Unallocated expenses

                   

Personnel expenses (including stock-based compensation)

    6,513     10,372     3,859  

Facility related and other

    4,477     2,651     (1,826 )

Total research and development expenses

  $ 21,039   $ 48,108   $ 27,069  

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