Nature of Business and Basis of Presentation	3 Months Ended
Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
NATURE OF BUSINESS AND BASIS OF PRESENTATION	1. NATURE OF BUSINESS AND BASIS OF PRESENTATION Nature of Business—Homology Medicines, Inc. (the “Company”) is a clinical-stage genetic medicines company dedicated to translating proprietary gene therapy and gene editing technology into novel treatments for patients with rare genetic diseases with significant unmet medical needs by curing the underlying cause of the disease. The Company was founded in March 2015 as a Delaware corporation. Its principal offices are in Bedford, Massachusetts. Since its inception, the Company has devoted substantially all of its resources to recruiting personnel, developing its technology platform and advancing its pipeline of product candidates, developing and implementing manufacturing processes, building out internal manufacturing and research and development space, and maintaining and building its intellectual property portfolio. The Company is subject to a number of risks similar to those of other companies conducting high-risk, early-stage research and development of product candidates. Principal among these risks are dependency on key individuals and intellectual property, competition from other products and companies, and the technical risks associated with the successful research, development and manufacturing of its product candidates. The Company’s success is dependent upon its ability to continue to raise additional capital in order to fund ongoing research and development, conduct clinical trials, obtain regulatory approval of its products, further expand its manufacturing capacity, successfully commercialize its products, generate revenue, meet its obligations, and, ultimately, attain profitable operations. On April 2, 2018, the Company completed its initial public offering (“IPO”) with the sale of 10,350,000 shares of its common stock, including shares issued upon the exercise in full of the underwriters’ over-allotment option, at a public offering price of $16.00 per share, resulting in net proceeds of $150.8 million, after deducting underwriting discounts and commissions and offering expenses. Upon the closing of the IPO, all of the Company’s outstanding shares of convertible preferred stock automatically converted into 24,168,656 shares of common stock at the applicable conversion ratio then in effect. On April 12, 2019, the Company completed a follow-on public offering of its common stock. The Company sold 5,555,556 shares of its common stock at a public offering price of $22.50 per share and received net proceeds of $116.9 million, after deducting underwriting discounts and commissions and offering expenses. In addition, on April 26, 2019 and May 7, 2019, in connection with the exercise in full of the underwriters’ option to purchase additional shares, the Company issued an aggregate of 833,333 shares of its common stock at a public offering price of $22.50 per share and received net proceeds of $17.6 million, after deducting underwriting discounts and commissions. On November 9, 2020, the Company entered into a common stock purchase agreement (the “Stock Purchase Agreement”) with Pfizer Inc. (“Pfizer”), pursuant to which the Company sold to Pfizer 5,000,000 shares of the Company’s common stock through a private placement transaction (the “Private Placement”) at a purchase price of $12.00 per share, for an aggregate purchase price of $60.0 million (see Note 11). On March 12, 2020, the Company filed a Registration Statement on Form S-3 (File No. 333-237131) (as amended, the “Shelf”) with the Securities and Exchange Commission (“SEC”) in relation to the registration of common stock, preferred stock, debt securities, warrants and/or units of any combination thereof for a period up to three years from the date of the filing. The Shelf became effective on March 12, 2020. The Company also simultaneously entered into a sales agreement with Cowen and Company, LLC (“Cowen”), as sales agent, providing for the offering, issuance and sale by the Company of up to an aggregate $150.0 million of its common stock from time to time in “at-the-market” offerings under the Shelf (the “ATM”). In connection with the filing of certain post-effective amendments to the Shelf, the sales agreement prospectus supplement now covers the offering, issuance and sale by the Company of up to an aggregate $148.4 million of its common stock. During the three months ended March 31, 2021, the Company sold 114,914 shares of common stock under the Sales Agreement, at an average price of approximately $14.00 per share, raising aggregate net proceeds of approximately $1.5 million after deducting an aggregate commission of 3% and issuance costs. As of March 31, 2021, there remained $148.4 million of common stock available for sale under the ATM. To date, the Company has not generated any revenue from product sales and does not expect to generate any revenue from the sale of product in the foreseeable future. Through March 31, 2021, the Company has financed its operations primarily through public offerings of its common stock, the issuance of convertible preferred stock, and with proceeds from its collaboration and license agreement with Novartis Institutes of BioMedical Research, Inc. (“Novartis”) (see Note 10) and its private placement with Pfizer. During the three months ended March 31, 2021, the Company incurred a net loss of $1.1 million and as of March 31, 2021, had $329.4 million in accumulated deficit. The Company expects to incur additional operating losses and negative operating cash flows for the foreseeable future. Based on current projections, management believes that cash and cash equivalents and short-term investments as of March 31, 2021, together with the approximately $49.4 million in net proceeds received from a follow-on offering of the Company’s common stock in April 2021 (see Note 12), will enable the Company to continue its operations into the first quarter of 2023. In the absence of a significant source of recurring revenue, the continued viability of the Company beyond that point is dependent on its ability to continue to raise additional capital to finance its operations. There can be no assurance that the Company will be able to obtain sufficient capital to cover its costs on acceptable terms, if at all. Basis of Presentation— The accompanying unaudited condensed consolidated financial statements have been prepared by the Company in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and pursuant to the rules and regulations of the SEC for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2020, included in the Company's Annual Report on Form 10-K on file with the SEC. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position as of March 31, 2021, and consolidated results of operations for the three months ended March 31, 2021 and 2020, and cash flows for the three months ended March 31, 2021 and 2020. Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021.

Nature of Business and Basis of Presentation

3 Months Ended

Mar. 31, 2021

Organization Consolidation And Presentation Of Financial Statements [Abstract]

NATURE OF BUSINESS AND BASIS OF PRESENTATION

1. NATURE OF BUSINESS AND BASIS OF PRESENTATION

Nature of Business—Homology Medicines, Inc. (the “Company”) is a clinical-stage genetic medicines company dedicated to translating proprietary gene therapy and gene editing technology into novel treatments for patients with rare genetic diseases with significant unmet medical needs by curing the underlying cause of the disease. The Company was founded in March 2015 as a Delaware corporation. Its principal offices are in Bedford, Massachusetts.

Since its inception, the Company has devoted substantially all of its resources to recruiting personnel, developing its technology platform and advancing its pipeline of product candidates, developing and implementing manufacturing processes, building out internal manufacturing and research and development space, and maintaining and building its intellectual property portfolio. The Company is subject to a number of risks similar to those of other companies conducting high-risk, early-stage research and development of product candidates. Principal among these risks are dependency on key individuals and intellectual property, competition from other products and companies, and the technical risks associated with the successful research, development and manufacturing of its product candidates. The Company’s success is dependent upon its ability to continue to raise additional capital in order to fund ongoing research and development, conduct clinical trials, obtain regulatory approval of its products, further expand its manufacturing capacity, successfully commercialize its products, generate revenue, meet its obligations, and, ultimately, attain profitable operations.

On April 2, 2018, the Company completed its initial public offering (“IPO”) with the sale of 10,350,000 shares of its common stock, including shares issued upon the exercise in full of the underwriters’ over-allotment option, at a public offering price of $16.00 per share, resulting in net proceeds of $150.8 million, after deducting underwriting discounts and commissions and offering expenses. Upon the closing of the IPO, all of the Company’s outstanding shares of convertible preferred stock automatically converted into 24,168,656 shares of common stock at the applicable conversion ratio then in effect.

On April 12, 2019, the Company completed a follow-on public offering of its common stock. The Company sold 5,555,556 shares of its common stock at a public offering price of $22.50 per share and received net proceeds of $116.9 million, after deducting underwriting discounts and commissions and offering expenses. In addition, on April 26, 2019 and May 7, 2019, in connection with the exercise in full of the underwriters’ option to purchase additional shares, the Company issued an aggregate of 833,333 shares of its common stock at a public offering price of $22.50 per share and received net proceeds of $17.6 million, after deducting underwriting discounts and commissions.

On November 9, 2020, the Company entered into a common stock purchase agreement (the “Stock Purchase Agreement”) with Pfizer Inc. (“Pfizer”), pursuant to which the Company sold to Pfizer 5,000,000 shares of the Company’s common stock through a private placement transaction (the “Private Placement”) at a purchase price of $12.00 per share, for an aggregate purchase price of $60.0 million (see Note 11).

On March 12, 2020, the Company filed a Registration Statement on Form S-3 (File No. 333-237131) (as amended, the “Shelf”) with the Securities and Exchange Commission (“SEC”) in relation to the registration of common stock, preferred stock, debt securities, warrants and/or units of any combination thereof for a period up to three years from the date of the filing. The Shelf became effective on March 12, 2020. The Company also simultaneously entered into a sales agreement with Cowen and Company, LLC (“Cowen”), as sales agent, providing for the offering, issuance and sale by the Company of up to an aggregate $150.0 million of its common stock from time to time in “at-the-market” offerings under the Shelf (the “ATM”). In connection with the filing of certain post-effective amendments to the Shelf, the sales agreement prospectus supplement now covers the offering, issuance and sale by the Company of up to an aggregate $148.4 million of its common stock. During the three months ended March 31, 2021, the Company sold 114,914 shares of common stock under the Sales Agreement, at an average price of approximately $14.00 per share, raising aggregate net proceeds of approximately $1.5 million after deducting an aggregate commission of 3% and issuance costs. As of March 31, 2021, there remained $148.4 million of common stock available for sale under the ATM.

To date, the Company has not generated any revenue from product sales and does not expect to generate any revenue from the sale of product in the foreseeable future. Through March 31, 2021, the Company has financed its operations primarily through public offerings of its common stock, the issuance of convertible preferred stock, and with proceeds from its collaboration and license agreement with Novartis Institutes of BioMedical Research, Inc. (“Novartis”) (see Note 10) and its private placement with Pfizer. During the three months ended March 31, 2021, the Company incurred a net loss of $1.1 million and as of March 31, 2021, had $329.4 million in accumulated deficit. The Company expects to incur additional operating losses and negative operating cash flows for the foreseeable future.

Based on current projections, management believes that cash and cash equivalents and short-term investments as of March 31, 2021, together with the approximately $49.4 million in net proceeds received from a follow-on offering of the Company’s common stock in April 2021 (see Note 12), will enable the Company to continue its operations into the first quarter of 2023. In the absence of a significant source of recurring revenue, the continued viability of the Company beyond that point is dependent on its ability to continue to raise additional capital to finance its operations. There can be no assurance that the Company will be able to obtain sufficient capital to cover its costs on acceptable terms, if at all.

Basis of Presentation— The accompanying unaudited condensed consolidated financial statements have been prepared by the Company in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and pursuant to the rules and regulations of the SEC for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2020, included in the Company's Annual Report on Form 10-K on file with the SEC.

The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position as of March 31, 2021, and consolidated results of operations for the three months ended March 31, 2021 and 2020, and cash flows for the three months ended March 31, 2021 and 2020. Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021.