Summary of Significant Accounting Policies	9 Months Ended
Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation These financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Certain amounts in accrued expenses reported in the Company's prior year condensed consolidated financial statements have been reclassified as accrued research costs to conform to the current year presentation. Unaudited Interim Condensed Financial Statements The interim condensed balance sheet as of September 30, 2021 and the condensed statements of operations and changes in redeemable convertible preferred stock and stockholders’ equity (deficit) for the three and nine months ended September 30, 2021 and 2020 and the condensed statements of cash flows for the nine months ended September 30, 2021 and 2020 are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the annual financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair statement of the Company’s financial position as of September 30, 2021 and its results of operations and cash flows for the nine months ended September 30, 2021 and 2020. The financial data and the other financial information disclosed in these notes to the financial statements related to the three-month and nine-month periods are also unaudited. The results of operations for the nine months ended September 30, 2021 are not necessarily indicative of the results to be expected for the year ending December 31, 2021 or for any other future annual or interim period. The condensed balance sheet as of December 31, 2020, included herein was derived from the audited financial statements as of that date. Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from these interim financial statements. These condensed financial statements should be read in conjunction with the Company's audited financial statements included in the prospectus dated June 24, 2021 that forms a part of the Company's Registration Statement on Form S-1 (File No. 333-256838), as filed with the SEC pursuant to Rule 424(b)(4) promulgated under the Securities Act of 1933, as amended. Significant Accounting Policies The significant accounting policies used in preparation of these condensed financial statements for the three and nine months ended September 30, 2021 are consistent with those discussed in Note 2 to the consolidated financial statements included in the Company’s Amendment No. 3 to Form S-1 filed on June 22, 2021, except as noted below and within the "Adopted and Recently Issued Accounting Pronouncements" section. Use of Estimates The preparation of condensed financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed financial statements, and the reported amounts of expenses during the reporting period. On an ongoing basis, the Company evaluates estimates and assumptions, including but not limited to those related to the fair value of the redeemable convertible preferred stock tranche liability, the fair value of redeemable convertible preferred stock and common stock, stock-based compensation expense, accruals for research and development costs, lease assets and liabilities, the valuation of deferred tax assets, and uncertain income tax positions. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Actual results may differ materially from those estimates. Leases Effective January 1, 2021, the Company adopted ASC Topic 842, Leases (“ASC 842”) using the optional transition method and applied the standard only to leases that existed at that date. Under the optional transition method, the Company does not need to restate the comparative periods in transition and will continue to present financial information and disclosures for periods before January 1, 2021 in accordance with ASC Topic 840. The Company has elected the package of practical expedients allowed under ASC Topic 842, which permits the Company to account for its existing operating leases as operating leases under the new guidance, without reassessing the Company’s prior conclusions about lease identification, lease classification and initial direct cost. As a result of the adoption of the new lease accounting guidance on January 1, 2021, the Company recognized no cumulative adjustment to accumulated deficit since the Company had only one operating lease with a term of less than 12 months and no plans to extend the lease. On January 27, 2021, the Company entered into a new lease agreement for office and lab space in South San Francisco, CA that included two office suites. The lease terms for the office suites commenced in July and August 2021, respectively. Upon commencement of the lease terms during July and August for the two office suites, the Company recognized operating lease right-of-use assets of $4.1 million and operating lease liabilities of $4.1 million (see Note 8). On August 30, 2021, the Company entered into a service agreement with The Laboratory for Cell and Gene Medicine (“LCGM”) that included an embedded operating lease as the manufacturing suit in LCGM’s facility is designated for the Company’s exclusive use though April 30, 2023. Upon commencement of the agreement, the Company recognized operating lease right-of-use assets of $5.3 million and operating lease liabilities of $5.3 million (see Notes 6 and 8). The Company determines the initial classification and measurement of its right-of-use assets and lease liabilities at the lease commencement date and thereafter if modified. The lease term includes any renewal options and termination options that the Company is reasonably assured to exercise. The present value of lease payments is determined by using the interest rate implicit in the lease, if that rate is readily determinable; otherwise, the Company uses its incremental borrowing rate. The incremental borrowing rate is determined by using the rate of interest that the Company would pay to borrow on a collateralized basis an amount equal to the lease payments for a similar term and in a similar economic environment. Fixed lease expense for operating leases is recognized on a straight-line basis, unless the right-of-use assets has been impaired, over the reasonably assured lease term based on the total lease payments and is included in operating expenses in the condensed statements of operations and comprehensive loss. Variable lease expenses are recognized as incurred. Fixed and variable lease expense on operating leases is recognized within operating expenses in the condensed statements of operations and comprehensive loss. For operating leases for which the right-of-use assets has been impaired, the Company will recognize the amortization of the right-of-use assets on a straight-line basis over the remaining lease term with lease expense included in operating expenses in the condensed statements of operations and comprehensive loss. For all leases, rent payments that are based on a fixed index or rate at the lease commencement date are included in the measurement of lease assets and lease liabilities at the lease commencement date. The Company has elected the practical expedient to not separate lease and non-lease components. The Company’s non-lease components are primarily related to maintenance, insurance and taxes, which varies based on future outcomes and is thus recognized in lease expense when incurred. Adopted and Recently Issued Accounting Pronouncements The Company is a smaller reporting company and an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay the adoption of new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. Thus, the Company has elected to use the extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that (i) the Company is no longer an emerging growth company or (ii) the Company affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. However as described below, the Company early adopted certain accounting standards, as the JOBS Act does not preclude an emerging growth company from adopting a new or revised accounting standard earlier than the time that such standard applies to private companies to the extent early adoption is permitted. In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases. Under ASC 842, the Company determines if an arrangement is a lease at inception. Lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. In determining the present value of lease payments, the Company uses its incremental borrowing rate based on the information available at the lease commencement date if the rate implicit in the lease is not readily determinable. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record right-of-use assets and lease liabilities for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today and are not recorded on the Company’s balance sheet. For non-public entities, ASU 2016-02 is effective for annual reporting periods beginning after December 15, 2021, including interim periods within those fiscal years, and early adoption is permitted. The Company early adopted the new standard as of January 1, 2021 on a modified retrospective basis with no cumulative adjustment to accumulated deficit since, as of the adoption date, the Company had only one operating lease, with a term of less than 12 months, and no plans to extend. The Company elected to take the practical expedient to not separate lease and non-lease components as part of the adoption. Lease agreements entered into after the adoption of Topic 842 that include lease and non-lease components are accounted for as a single lease component. Beginning on January 1, 2021, the Company’s operating leases, excluding those with terms less than 12 months, will be discounted and recorded as assets and liabilities on the Company’s balance sheet. As of September 30, 2021, the Company had operating lease right-of-use assets of $9.1 million and operating lease liabilities of $9.5 million related to the lease recorded on its condensed balance sheets. In June 2016, the FASB issued ASU 2016-13, Credit Losses. The FASB also issued amendments and the initial ASU, and all updates are included herein as the Credit Losses standard or Topic 326. The new standard generally applies to financial assets and requires those assets to be reported at the amount expected to be realized. The ASU is effective for fiscal years beginning after December 15, 2022 and interim periods within those fiscal years. The Company is currently evaluating the potential impact of this standard on its condensed financial statements. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740). The amendments in ASU 2019-12 simplify the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The amendments also improve consistent application of and simplify U.S. GAAP or other areas of Topic 740 by clarifying and amending existing guidance. The new standard is effective for the Company on January 1, 2022 and for interim periods beginning on January 1, 2023. The Company is currently evaluating the potential impact of this standard on its condensed financial statements. In August 2020, the FASB issued ASU No. 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (ASU 2020-06), which simplifies the accounting for convertible instruments by reducing the number of accounting models available for convertible debt instruments. This guidance also eliminates the treasury stock method to calculate diluted earnings per share for convertible instruments and requires the use of the if-converted method. This guidance will be effective for the Company in the first quarter of 2022 on a full or modified retrospective basis, with early adoption permitted. The Company is currently evaluating the potential impact of this standard on its condensed financial statements.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2021

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

These financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”).

Certain amounts in accrued expenses reported in the Company's prior year condensed consolidated financial statements have been reclassified as accrued research costs to conform to the current year presentation.

Unaudited Interim Condensed Financial Statements

The interim condensed balance sheet as of September 30, 2021 and the condensed statements of operations and changes in redeemable convertible preferred stock and stockholders’ equity (deficit) for the three and nine months ended September 30, 2021 and 2020 and the condensed statements of cash flows for the nine months ended September 30, 2021 and 2020 are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the annual financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair statement of the Company’s financial position as of September 30, 2021 and its results of operations and cash flows for the nine months ended September 30, 2021 and 2020. The financial data and the other financial information disclosed in these notes to the financial statements related to the three-month and nine-month periods are also unaudited. The results of operations for the nine months ended September 30, 2021 are not necessarily indicative of the results to be expected for the year ending December 31, 2021 or for any other future annual or interim period. The condensed balance sheet as of December 31, 2020, included herein was derived from the audited financial statements as of that date. Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from these interim financial statements. These condensed financial statements should be read in conjunction

with the Company's audited financial statements included in the prospectus dated June 24, 2021 that forms a part of the Company's Registration Statement on Form S-1 (File No. 333-256838), as filed with the SEC pursuant to Rule 424(b)(4) promulgated under the Securities Act of 1933, as amended.

Significant Accounting Policies

The significant accounting policies used in preparation of these condensed financial statements for the three and nine months ended September 30, 2021 are consistent with those discussed in Note 2 to the consolidated financial statements included in the Company’s Amendment No. 3 to Form S-1 filed on June 22, 2021, except as noted below and within the "Adopted and Recently Issued Accounting Pronouncements" section.

Use of Estimates

The preparation of condensed financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed financial statements, and the reported amounts of expenses during the reporting period. On an ongoing basis, the Company evaluates estimates and assumptions, including but not limited to those related to the fair value of the redeemable convertible preferred stock tranche liability, the fair value of redeemable convertible preferred stock and common stock, stock-based compensation expense, accruals for research and development costs, lease assets and liabilities, the valuation of deferred tax assets, and uncertain income tax positions. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Actual results may differ materially from those estimates.

Leases

Effective January 1, 2021, the Company adopted ASC Topic 842, Leases (“ASC 842”) using the optional transition method and applied the standard only to leases that existed at that date. Under the optional transition method, the Company does not need to restate the comparative periods in transition and will continue to present financial information and disclosures for periods before January 1, 2021 in accordance with ASC Topic 840. The Company has elected the package of practical expedients allowed under ASC Topic 842, which permits the Company to account for its existing operating leases as operating leases under the new guidance, without reassessing the Company’s prior conclusions about lease identification, lease classification and initial direct cost. As a result of the adoption of the new lease accounting guidance on January 1, 2021, the Company recognized no cumulative adjustment to accumulated deficit since the Company had only one operating lease with a term of less than 12 months and no plans to extend the lease.

On January 27, 2021, the Company entered into a new lease agreement for office and lab space in South San Francisco, CA that included two office suites. The lease terms for the office suites commenced in July and August 2021, respectively. Upon commencement of the lease terms during July and August for the two office suites, the Company recognized operating lease right-of-use assets of $4.1 million and operating lease liabilities of $4.1 million (see Note 8).

On August 30, 2021, the Company entered into a service agreement with The Laboratory for Cell and Gene Medicine (“LCGM”) that included an embedded operating lease as the manufacturing suit in LCGM’s facility is designated for the Company’s exclusive use though April 30, 2023. Upon commencement of the agreement, the Company recognized operating lease right-of-use assets of $5.3 million and operating lease liabilities of $5.3 million (see Notes 6 and 8).

The Company determines the initial classification and measurement of its right-of-use assets and lease liabilities at the lease commencement date and thereafter if modified. The lease term includes any renewal options and termination options that the Company is reasonably assured to exercise. The present value of lease payments is determined by using the interest rate implicit in the lease, if that rate is readily determinable; otherwise, the Company uses its incremental borrowing rate. The incremental borrowing rate is determined by using the rate of interest that the Company would pay to borrow on a collateralized basis an amount equal to the lease payments for a similar term and in a similar economic environment.

Fixed lease expense for operating leases is recognized on a straight-line basis, unless the right-of-use assets has been impaired, over the reasonably assured lease term based on the total lease payments and is included in operating expenses in the condensed statements of operations and comprehensive loss. Variable lease expenses are recognized as incurred. Fixed and variable lease expense on operating leases is recognized within operating expenses in the condensed statements of operations and comprehensive loss.

For operating leases for which the right-of-use assets has been impaired, the Company will recognize the amortization of the right-of-use assets on a straight-line basis over the remaining lease term with lease expense included in operating expenses in the condensed statements of operations and comprehensive loss.

For all leases, rent payments that are based on a fixed index or rate at the lease commencement date are included in the measurement of lease assets and lease liabilities at the lease commencement date.

The Company has elected the practical expedient to not separate lease and non-lease components. The Company’s non-lease components are primarily related to maintenance, insurance and taxes, which varies based on future outcomes and is thus recognized in lease expense when incurred.

Adopted and Recently Issued Accounting Pronouncements

The Company is a smaller reporting company and an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay the adoption of new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. Thus, the Company has elected to use the extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that (i) the Company is no longer an emerging growth company or (ii) the Company affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. However as described below, the Company early adopted certain accounting standards, as the JOBS Act does not preclude an emerging growth company from adopting a new or revised accounting standard earlier than the time that such standard applies to private companies to the extent early adoption is permitted.

In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases. Under ASC 842, the Company determines if an arrangement is a lease at inception. Lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. In determining the present value of lease payments, the Company uses its incremental borrowing rate based on the information available at the lease commencement date if the rate implicit in the lease is not readily determinable. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record right-of-use assets and lease liabilities for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today and are not recorded on the Company’s balance sheet. For non-public entities, ASU 2016-02 is effective for annual reporting periods beginning after December 15, 2021, including interim periods within those fiscal years, and early adoption is permitted. The Company early adopted the new standard as of January 1, 2021 on a modified retrospective basis with no cumulative adjustment to accumulated deficit since, as of the adoption date, the Company had only one operating lease, with a term of less than 12 months, and no plans to extend. The Company elected to take the practical expedient to not separate lease and non-lease components as part of the adoption. Lease agreements entered into after the adoption of Topic 842 that include lease and non-lease components are accounted for as a single lease component. Beginning on January 1, 2021, the Company’s operating leases, excluding those with terms less than 12 months, will be discounted and recorded as assets and liabilities on the Company’s balance sheet. As of September 30, 2021, the Company had operating lease right-of-use assets of $9.1 million and operating lease liabilities of $9.5 million related to the lease recorded on its condensed balance sheets.

In June 2016, the FASB issued ASU 2016-13, Credit Losses. The FASB also issued amendments and the initial ASU, and all updates are included herein as the Credit Losses standard or Topic 326. The new standard generally applies to financial assets and requires those assets to be reported at the amount expected to be realized. The ASU is effective for fiscal years beginning after December 15, 2022 and interim periods within those fiscal years. The Company is currently evaluating the potential impact of this standard on its condensed financial statements.

In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740). The amendments in ASU 2019-12 simplify the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The amendments also improve consistent application of and simplify U.S. GAAP or other areas of Topic 740 by clarifying and amending existing guidance. The new standard is effective for the Company on January 1, 2022 and for interim periods beginning on January 1, 2023. The Company is currently evaluating the potential impact of this standard on its condensed financial statements.

In August 2020, the FASB issued ASU No. 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (ASU 2020-06), which simplifies the accounting for convertible instruments by reducing the number of accounting models available for convertible debt instruments. This guidance also eliminates the treasury stock method to calculate diluted earnings per share for convertible instruments and requires the use of the if-converted method. This guidance will be effective for the Company in the first quarter of 2022 on a full or modified retrospective basis, with early adoption permitted. The Company is currently evaluating the potential impact of this standard on its condensed financial statements.