Q32 Bio (QTTB) 10-Q 12 Aug 21 2021 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2021	Aug. 02, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2021
Document Fiscal Period Focus	Q2
Document Fiscal Year Focus	2021
Entity Registrant Name	Homology Medicines, Inc
Entity Central Index Key	0001661998
Trading Symbol	FIXX
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		57,088,143
Entity File Number	001-38433
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-3468154
Entity Address, Address Line One	One Patriots Park
Entity Address, City or Town	Bedford
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	01730
City Area Code	781
Local Phone Number	301-7277
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Security Exchange Name	NASDAQ
Document Quarterly Report	true
Document Transition Report	false

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 116,313,000	$ 217,431,000
Short-term investments	96,980,000	0
Prepaid expenses and other current assets	4,644,000	2,133,000
Total current assets	217,937,000	219,564,000
Property and equipment, net	34,379,000	37,002,000
Right-of-use assets	5,328,000	5,897,000
Restricted cash	1,274,000	1,274,000
Total assets	258,918,000	263,737,000
Current liabilities:
Accounts payable	5,696,000	4,722,000
Accrued expenses and other liabilities	7,689,000	9,803,000
Operating lease liabilities	2,142,000	2,501,000
Deferred revenue	4,007,000	5,632,000
Total current liabilities	19,534,000	22,658,000
Non-current liabilities:
Operating lease liabilities, net of current portion	12,047,000	12,941,000
Deferred revenue, net of current portion	2,760,000	32,143,000
Total liabilities	34,341,000	67,742,000
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 10,000,000 shares authorized; no shares issued and outstanding at June 30, 2021 and December 31, 2020, respectively
Common stock, $0.0001 par value; 200,000,000 shares authorized; 57,088,026 and 50,268,666 shares issued as of June 30, 2021 and December 31, 2020, respectively; and 57,086,957 and 50,265,575 shares outstanding as of June 30, 2021 and December 31, 2020, respectively	6,000	5,000
Additional paid-in capital	584,510,000	524,358,000
Accumulated other comprehensive loss	2,000
Accumulated deficit	(359,941,000)	(328,368,000)
Total stockholders’ equity	224,577,000	195,995,000
Total liabilities and stockholders' equity	$ 258,918,000	$ 263,737,000

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Jun. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	57,088,026	50,268,666
Common stock, shares outstanding	57,086,957	50,265,575

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue	$ 2,187	$ 567	$ 31,492	$ 1,155
Operating expenses:
Research and development	23,697	27,471	45,452	56,780
General and administrative	9,042	8,793	17,703	16,563
Total operating expenses	32,739	36,264	63,155	73,343
Loss from operations	(30,552)	(35,697)	(31,663)	(72,188)
Other income:
Interest income	52	357	90	1,517
Total other income	52	357	90	1,517
Net loss	$ (30,500)	$ (35,340)	$ (31,573)	$ (70,671)
Net loss per share-basic and diluted	$ (0.54)	$ (0.78)	$ (0.59)	$ (1.56)
Weighted-average common shares outstanding-basic and diluted	56,497,461	45,207,934	53,429,634	45,180,096

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (30,500)	$ (35,340)	$ (31,573)	$ (70,671)
Other comprehensive (loss):
Change in unrealized gain (loss) on available for sale securities, net	25	30	2	(172)
Total other comprehensive loss	25	30	2	(172)
Comprehensive loss	$ (30,475)	$ (35,310)	$ (31,571)	$ (70,843)

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Follow-on Offering [Member]	ATM	Common Stock	Common StockFollow-on Offering [Member]	Common StockATM	Additional Paid-in Capital	Additional Paid-in CapitalFollow-on Offering [Member]	Additional Paid-in CapitalATM	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2019	$ 258,507			$ 4			$ 457,994			$ 183	$ (199,674)
Beginning balance, Shares at Dec. 31, 2019				45,116,742
Vesting of common stock from option exercises	12						12
Vesting of common stock from option exercises, Shares				15,542
Issuance of common stock from option exercises	135						135
Issuance of common stock from option exercises, Shares				34,311
Issuance of common stock pursuant to employee stock purchase plan	537						537
Issuance of common stock pursuant to employee stock purchase plan, Shares				39,471
Stock-based compensation	2,883						2,883
Other comprehensive gain (loss)	(202)									(202)
Net loss	(35,331)										(35,331)
Ending balance at Mar. 31, 2020	226,541			$ 4			461,561			(19)	(235,005)
Ending balance, Shares at Mar. 31, 2020				45,206,066
Beginning balance at Dec. 31, 2019	258,507			$ 4			457,994			183	(199,674)
Beginning balance, Shares at Dec. 31, 2019				45,116,742
Other comprehensive gain (loss)	(172)
Net loss	(70,671)
Ending balance at Jun. 30, 2020	194,704			$ 4			465,034			11	(270,345)
Ending balance, Shares at Jun. 30, 2020				45,210,653
Beginning balance at Mar. 31, 2020	226,541			$ 4			461,561			(19)	(235,005)
Beginning balance, Shares at Mar. 31, 2020				45,206,066
Vesting of common stock from option exercises	4						4
Vesting of common stock from option exercises, Shares				1,011
Issuance of common stock from option exercises	16						16
Issuance of common stock from option exercises, Shares				3,576
Stock-based compensation	3,453						3,453
Other comprehensive gain (loss)	30									30
Net loss	(35,340)										(35,340)
Ending balance at Jun. 30, 2020	194,704			$ 4			465,034			11	(270,345)
Ending balance, Shares at Jun. 30, 2020				45,210,653
Beginning balance at Dec. 31, 2020	195,995			$ 5			524,358				(328,368)
Beginning balance, Shares at Dec. 31, 2020				50,265,575
Issuance of common stock, net of discounts and issuance costs			$ 1,454						$ 1,454
Issuance of common stock, net of discounts and issuance costs, Shares						114,914
Vesting of common stock from option exercises	4						4
Vesting of common stock from option exercises, Shares				1,011
Issuance of common stock from option exercises	143						143
Issuance of common stock from option exercises, Shares				55,277
Issuance of common stock pursuant to employee stock purchase plan	446						446
Issuance of common stock pursuant to employee stock purchase plan, Shares				48,792
Stock-based compensation	3,883						3,883
Other comprehensive gain (loss)	(23)									(23)
Net loss	(1,073)										(1,073)
Ending balance at Mar. 31, 2021	200,829			$ 5			530,288			(23)	(329,441)
Ending balance, Shares at Mar. 31, 2021				50,485,569
Beginning balance at Dec. 31, 2020	$ 195,995			$ 5			524,358				(328,368)
Beginning balance, Shares at Dec. 31, 2020				50,265,575
Issuance of common stock, net of discounts and issuance costs, Shares			114,914
Issuance of common stock from option exercises, Shares	59,348
Other comprehensive gain (loss)	$ 2
Net loss	(31,573)
Ending balance at Jun. 30, 2021	224,577			$ 6			584,510			2	(359,941)
Ending balance, Shares at Jun. 30, 2021				57,086,957
Beginning balance at Mar. 31, 2021	200,829			$ 5			530,288			(23)	(329,441)
Beginning balance, Shares at Mar. 31, 2021				50,485,569
Issuance of common stock, net of discounts and issuance costs		$ 49,744			$ 1			$ 49,743
Issuance of common stock, net of discounts and issuance costs, Shares					6,596,306
Vesting of common stock from option exercises	4						4
Vesting of common stock from option exercises, Shares				1,011
Issuance of common stock from option exercises	2						2
Issuance of common stock from option exercises, Shares				4,071
Stock-based compensation	4,473						4,473
Other comprehensive gain (loss)	25									25
Net loss	(30,500)										(30,500)
Ending balance at Jun. 30, 2021	$ 224,577			$ 6			$ 584,510			$ 2	$ (359,941)
Ending balance, Shares at Jun. 30, 2021				57,086,957

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities:
Net loss	$ (31,573)	$ (70,671)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	4,181	3,899
Noncash lease expense	569	450
Stock-based compensation expense	8,356	6,336
Accretion of discount (amortization of premium) on short-term investments	414	(193)
Loss on disposal of property and equipment	6
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,511)	1,025
Accounts payable	991	4,920
Accrued expenses and other liabilities	(2,106)	1,888
Deferred revenue	(31,008)	(397)
Operating lease liabilities	(1,253)	(1,090)
Net cash used in operating activities	(53,934)	(53,833)
Cash flows from investing activities:
Purchases of short-term investments	(97,392)	(19,992)
Maturities of short-term investments		189,620
Purchases of property and equipment	(1,581)	(2,621)
Net cash (used in) provided by investing activities	(98,973)	167,007
Cash flows from financing activities:
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs	49,744
Proceeds from issuance of common stock pursuant to employee stock purchase plan	446	537
Proceeds from issuance of common stock from option exercises	145	151
Net cash provided by financing activities	51,789	688
Net change in cash, cash equivalents and restricted cash	(101,118)	113,862
Cash, cash equivalents and restricted cash, beginning of period	218,705	55,048
Cash, cash equivalents and restricted cash, end of period	117,587	168,910
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	8	16
Property and equipment additions included in accounts payable	94	30
Property and equipment additions included in accrued expenses and other liabilities		7
Unrealized gain (loss) on available for sale securities, net	2	$ (172)
A T M
Cash flows from financing activities:
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs	$ 1,454

Nature of Business and Basis of

Nature of Business and Basis of Presentation	6 Months Ended
	Jun. 30, 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 April 6, 2021, the Company completed a follow-on public offering of its common stock. The Company sold 6,596,306 shares of its common stock at a price of $7.58 per share and received net proceeds of $49.7 million, after deducting offering expenses of $0.3 million. Under the terms of the underwriting agreement, the Company also granted the underwriter an option exercisable for 30 days to purchase up to an additional 989,445 shares of its common stock at a price of $7.58 per share. The underwriters did not exercise this option. The offering closed on April 9, 2021. The shares were sold pursuant to the Company’s effective shelf registration statement on Form S-3, as amended, and a related prospectus supplement filed with the SEC on April 8, 2021. 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 six months ended June 30, 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 $ million after deducting an aggregate commission of 3 % and issuance costs of $ 0.1 million . A s of June 30, 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 June 30, 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 six months ended June 30, 2021, the Company incurred a net loss of $31.6 million and as of June 30, 2021, had $359.9 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 June 30, 2021 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 June 30, 2021, and consolidated results of operations for the three and six months ended June 30, 2021 and 2020, and cash flows for the six months ended June 30, 2021 and 2020. Such adjustments are of a normal and recurring nature. The results of operations for the three and six months ended June 30, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Principles of Consolidation —The Company’s condensed consolidated financial statements include the accounts of the Company and its subsidiary, Homology Medicines Securities Corporation, a wholly owned Massachusetts corporation, for the sole purpose of buying, selling, and holding securities on the Company’s behalf. All intercompany balances and transactions have been eliminated in the condensed consolidated financial statements. Use of Estimates —The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenue, and expenses, and the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases its estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, revenue recognition, accrued research and development expenses and useful lives assigned to property and equipment. The Company assesses estimates on an ongoing basis; however, actual results could materially differ from those estimates. Comprehensive Income (Loss) —Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. The Company’s only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments. Cash and Cash Equivalents and Restricted Cash —Cash and cash equivalents consist of standard checking accounts, money market accounts and certain investments. The Company considers all highly liquid investments with original or remaining maturities at the time of purchase of 90 days or less to be cash equivalents. Restricted cash consists of cash serving as collateral for letters of credit issued for security deposits for the Company’s facility leases in Bedford, Massachusetts. The following table provides a reconciliation of cash, cash equivalents and restricted cash to amounts shown in the condensed consolidated statements of cash flows: June 30, 2021 2020 (in thousands) Cash and cash equivalents $ 116,313 $ 167,636 Restricted cash 1,274 1,274 Total cash, cash equivalents and restricted cash $ 117,587 $ 168,910 Short-Term Investments —Short-term investments represent holdings of available-for-sale marketable securities in accordance with the Company’s investment policy and cash management strategy. Short-term investments have maturities of greater than 90 days at the time of purchase and mature within one year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses reported within accumulated other comprehensive income as a separate component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the underlying security. Such amortization and accretion, together with interest on securities, are included in interest income in the Company’s condensed consolidated statements of operations. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other income. Offering Costs —The Company capitalizes incremental legal, professional accounting and other third-party fees that are directly associated with equity financings as other current assets until the transactions are completed. After equity financings are complete, these costs are recorded in stockholders’ equity as a reduction of additional paid-in capital generated as a result of the offering. Leases —The Company determines if an arrangement is a lease at contract inception. The Company’s contracts are determined to contain a lease when all of the following criteria based on the specific circumstances of the arrangement are met: (1) there is an identified asset for which there are no substantive substitution rights; (2) the Company has the right to obtain substantially all of the economic benefits from the identified asset; and (3) the Company has the right to direct the use of the identified asset. At the commencement date, operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of future lease payments over the expected lease term. The Company’s lease agreements do not provide an implicit rate. As a result, the Company utilizes an estimated incremental borrowing rate to discount lease payments, which is based on the rate of interest the Company would have to pay to borrow a similar amount on a collateralized basis over a similar term. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or lease incentives received. Operating lease cost is recognized over the expected term on a straight-line basis. The expected lease term includes noncancelable lease periods and, when applicable, periods covered by an option to extend the lease if the Company is reasonably certain to exercise that option, as well as periods covered by an option to terminate the lease if the Company is reasonably certain not to exercise that option. Variable lease cost is recognized as incurred. The Company acts as sublessor related to a sublease of the Company's former headquarters. Fixed sublease payments received are recognized on a straight-line basis over the sublease term and recorded as a reduction to lease cost. Right-of-use assets are periodically evaluated for impairment. Research and Development Costs —Research and development costs are charged to expense as incurred. Research and development expense consists of expenses incurred in performing research and development activities, including salaries and benefits, materials and supplies, preclinical and clinical expenses, stock-based compensation expense, depreciation of equipment, contract services, and other outside expenses. Costs for certain development activities are recognized based on an evaluation of the progress to completion of specific tasks using information provided to the Company by its vendors on their actual costs incurred. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid expense or accrued research and development expense. Revenue Recognition— Revenue is recognized in accordance with FASB Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”). On January 1, 2019, the Company adopted ASC 606 using the full retrospective transition method. Under ASC 606, the Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations , including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in the Company’s arrangements would likely consist of a license, rights to the Company’s intellectual property or research, development and manufacturing services. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of consideration to which the Company expects to be entitled to. The Company utilizes either the most likely amount method or expected value method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration that is included in the transaction price may be constrained and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts may include development and regulatory milestone payments that are assessed under the most likely amount method and constrained until it is probable that a significant revenue reversal would not occur. Milestone payments that are not within the Company’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such development and regulatory milestones and any related constraint, and if necessary, adjust its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from the Company’s collaboration arrangement. The Company allocates the transaction price based on the estimated standalone selling price of each performance obligation. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress for its over-time arrangements at each reporting period and, if necessary, updates the measure of progress and revenue recognized. Net Loss per Share— Basic net loss per share is computed by dividing net loss by the weighted-average number of common shares outstanding during the period. Diluted net loss per share is computed using the weighted-average number of common shares outstanding during the period and, if dilutive, the weighted-average number of potential shares of common stock. The weighted-average number of common shares included in the computation of diluted net loss gives effect to all potentially dilutive common equivalent shares, including outstanding stock options, restricted stock units and unvested shares of common stock. Common stock equivalent shares are excluded from the computation of diluted net loss per share if their effect is antidilutive. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is generally the same as basic net loss per share attributable to common stockholders , since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Recent Accounting Pronouncements —The Jumpstart Our Business Startups Act of 2012 permits an emerging growth company to take advantage of an extended transition period to comply with new or revised accounting standards applicable to public companies until those standards would otherwise apply to private companies. As an emerging growth company, the Company has elected to take advantage of this extended transition period. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In October 2020, the FASB issued ASU 2020-10, Codification Improvements All other new accounting pronouncements issued, but not yet effective or adopted have been deemed to be not relevant to the Company and, accordingly, are not expected to have a material impact once adopted.

Short-Term Investments

Short-Term Investments	6 Months Ended
	Jun. 30, 2021
Investments Disclosure [Abstract]
SHORT-TERM INVESTMENTS	3. SHORT-TERM INVESTMENTS The Company may invest its excess cash in fixed income instruments denominated and payable in U.S. dollars, including U.S. treasury securities, commercial paper, corporate debt securities and asset-backed securities in accordance with the Company’s investment policy that primarily seeks to maintain adequate liquidity and preserve capital. The Company has designated all investments as available-for-sale and therefore such investments are reported at fair value and classified as short-term investments on the Company’s condensed consolidated balance sheets. Unrealized gains or losses on investments are recorded in accumulated other comprehensive income or loss, a component of stockholders’ equity, on the Company’s condensed consolidated balance sheets. The following table summarizes the Company’s short-term investments as of June 30, 2021: As of June 30, 2021 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Commercial paper $ 61,944 $ — $ — $ 61,944 Corporate debt securities 35,034 4 (2 ) 35,036 Total $ 96,978 $ 4 $ (2 ) $ 96,980 The Company had no short-term investments as of December 31, 2020. The Company utilizes the specific identification method in computing realized gains and losses. The Company had no realized gains and losses on its available-for-sale securities for the three and six months ended June 30, 2021 and 2020. The contractual maturity dates of all of the Company’s investments are less than one year.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2021
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	4. FAIR VALUE MEASUREMENTS The Company’s financial instruments consist of cash and cash equivalents, short-term investments, restricted cash and accounts payable. The carrying amount of cash, restricted cash and accounts payable are each considered a reasonable estimate of fair value due to the short-term maturity. Assets measured at fair value on a recurring basis were as follows: Description June 30, 2021 Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Money market mutual funds $ 116,063 $ 116,063 $ — $ — Total cash equivalents $ 116,063 $ 116,063 $ — $ — Short-term investments: Commercial paper $ 61,944 $ — $ 61,944 $ — Corporate debt securities 35,036 — 35,036 — Total short-term investments $ 96,980 $ — $ 96,980 $ — Total financial assets $ 213,043 $ 116,063 $ 96,980 $ — Description December 31, 2020 Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Money market mutual funds $ 217,181 $ 217,181 $ — $ — Total financial assets $ 217,181 $ 217,181 $ — $ — Short-term securities are valued using models or other valuation methodologies that use Level 2 inputs. These models are primarily industry-standard models that consider various assumptions, including time value, yield curve, volatility factors, default rates, current market and contractual prices for the underlying financial instruments, as well as other relevant economic measures. Substantially all of these assumptions are observable in the marketplace, can be derived from observable data or are supported by observable levels at which transactions are executed in the marketplace. There were no transfers between fair value measure levels during the three and six months ended June 30, 2021 and 2020.

Property and Equipment, Net

Property and Equipment, Net	6 Months Ended
	Jun. 30, 2021
Property Plant And Equipment [Abstract]
PROPERTY AND EQUIPMENT, NET	5. PROPERTY AND EQUIPMENT, NET Property and equipment, net consists of the following: As of June 30, 2021 December 31, 2020 (in thousands) Laboratory equipment $ 13,752 $ 12,703 Manufacturing equipment 7,877 7,754 Computers and purchased software 1,597 1,596 Furniture and fixtures 1,625 1,363 Leasehold improvements 29,961 29,961 Assets not yet in service 63 — Property and equipment, at cost 54,875 53,377 Less: accumulated depreciation and amortization (20,496 ) (16,375 ) Property and equipment, net $ 34,379 $ 37,002 Depreciation expense for the three and six months ended June 30, 2021 was approximately $2.1 million and $4.2 million, respectively, compared to $2.0 million and $3.9 million, respectively for the three and six months ended June 30, 2020. The Company disposed of less than $0.1 million of property and equipment during the three and six months ended June 30, 2021. There were no disposals of property and equipment during the three and six months ended June 30, 2020.

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	6 Months Ended
	Jun. 30, 2021
Payables And Accruals [Abstract]
ACCRUED EXPENSES AND OTHER LIABILITIES	6. ACCRUED EXPENSES AND OTHER LIABILITIES Accrued expenses and other liabilities consist of the following: As of June 30, 2021 December 31, 2020 (in thousands) Accrued compensation and benefits $ 5,254 $ 6,902 Accrued research and development expenses 1,858 2,393 Accrued professional fees 397 291 Accrued other 180 217 Total accrued expenses and other liabilities $ 7,689 $ 9,803

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	7. COMMITMENTS AND CONTINGENCIES Operating Leases —In September 2016, the Company entered into a noncancelable operating lease beginning in November 2016 for office, laboratory and manufacturing space in Bedford, Massachusetts, that expires in October 2021, with an option for an additional three-year In December 2017, the Company entered into a noncancelable operating lease for approximately 67,000 square feet of research and development, manufacturing and general office space in Bedford, Massachusetts. The lease expires in February 2027 with an option for an additional five-year The Company maintains letters of credit, secured by restricted cash, for security deposits totaling $1.3 million as of June 30, 2021 and December 31, 2020, respectively, in conjunction with its current leases. The following table summarizes operating lease costs, variable lease costs and sublease income: Six months ended June 30, 2021 2020 (in thousands) Operating lease costs $ 1,246 $ 1,246 Variable lease costs 1,032 1,029 Sublease income (470 ) (457 ) Net lease cost $ 1,808 $ 1,818 The maturities of our operating lease liabilities and minimum lease payments as of June 30, 2021 were as follows: For the Years Ending December 31, Amount (in thousands) 2021 1,892 2022 2,987 2023 3,077 2024 3,169 Thereafter 7,197 Total undiscounted lease payments $ 18,322 Less: imputed interest (4,133 ) Present value of operating lease liabilities $ 14,189 The following table summarizes the lease term and discount rate as of June 30, 2021: As of June 30, 2021 Weighted-average remaining lease term (years) Operating leases 5.5 Weighted-average discount rate Operating leases 10.0 % The following table summarizes cash paid for amounts included in the measurement of the Company’s operating lease liabilities. Six months ended June 30, 2021 2020 (in thousands) Cash paid for amounts included in the measurement of lease liabilities $ 1,930 $ 1,886

Stock Incentive Plans

Stock Incentive Plans	6 Months Ended
	Jun. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
STOCK INCENTIVE PLANS	8. STOCK INCENTIVE PLANS 2015 Stock Incentive Plan In December 2015, the Company’s Board of Directors adopted the 2015 Stock Incentive Plan (the “2015 Plan”), which provided for the grant of qualified incentive and nonqualified stock options or restricted stock awards to the Company’s employees, officers, directors, advisors, and outside consultants. Stock options generally vest over a four-year 2018 Incentive Award Plan In March 2018, the Company’s Board of Directors adopted and the Company’s stockholders approved the Homology Medicines, Inc. 2018 Incentive Award Plan (the “2018 Plan” and, together with the 2015 Plan, the “Plans”), which became effective on the day prior to the first public trading date of the Company’s common stock. Upon effectiveness of the 2018 Plan, the Company ceased granting new awards under the 2015 Plan. The 2018 Plan provides for the grant of incentive stock options, nonqualified stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock or cash-based awards to employees and consultants of the Company and certain affiliates and directors of the Company. The number of shares of common stock initially available for issuance under the 2018 Plan was 3,186,205 shares of common stock plus the number of shares subject to awards outstanding under the 2015 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company on or after the effective date of the 2018 Plan. In addition, the number of shares of common stock available for issuance under the 2018 Plan is subject to an annual increase on the first day of each calendar year beginning on January 1, 2019, and ending on and including January 1, 2028, equal to the lesser of (i) 4% of the Company’s outstanding shares of common stock on the final day of the immediately preceding calendar year and (ii) such smaller number of shares of common stock as determined by the Company’s Board of Directors, provided that not more than 20,887,347 shares of common stock may be issued under the 2018 Plan upon the exercise of incentive stock options. Therefore, on January 1, 2021, an additional 2,010,623 shares were added to the 2018 Plan, representing 4% of total common shares outstanding at December 31, 2020. As of June 30, 2021, there were 2,390,494 shares available for future grant under the 2018 Plan. 2018 Employee Stock Purchase Plan In March 2018, the Company’s Board of Directors adopted and the Company’s stockholders approved the Homology Medicines, Inc. 2018 Employee Stock Purchase Plan (the “2018 ESPP”). The 2018 ESPP allows employees to buy Company stock through after-tax payroll deductions at a discount from market value. The 2018 ESPP is intended to qualify as an “employee stock purchase plan” under Section 423 of the Internal Revenue Code. The number of shares of common stock initially available for issuance under the 2018 ESPP was 353,980 shares of common stock plus an annual increase on the first day of each calendar year, beginning on January 1, 2019, and ending on and including January 1, 2028 equal to the lesser of (i) 1% of the Company’s outstanding shares of common stock on the final day of the immediately preceding calendar year and (ii) such smaller number of shares of common stock as determined by the Company’s Board of Directors, provided that not more than 4,778,738 shares of common stock may be issued under the 2018 ESPP. Therefore, on January 1, 2021, an additional 502,655 shares were added to the 2018 ESPP, representing 1% of total common shares outstanding at December 31, 2020. As of June 30, 2021, there were 1,493,513 shares available for future issuance under the 2018 ESPP. Under the 2018 ESPP, employees may purchase common stock through after-tax payroll deductions at a price equal to 85% of the lower of the fair market value on the first trading day of an offering period or the last trading day of an offering period. The 2018 ESPP generally provides for offering periods of six months in duration that end on the final trading day of each February and August. In accordance with the Internal Revenue Code, no employee will be permitted to accrue the right to purchase stock under the 2018 ESPP at a rate in excess of $25,000 worth of shares during any calendar year during which such a purchase right is outstanding (based on the fair market value per share of the Company’s common stock as of the first day of the offering period). During the six months ended June 30, 2021, 48,792 shares were issued under the 2018 ESPP for aggregate proceeds to the Company of $0.4 million. During the six months ended June 30, 2020, 39,471 shares were issued under the 2018 ESPP for aggregate proceeds to the Company of $0.5 million. Pursuant to the 2018 ESPP, the Company recorded stock-based compensation of $0.1 million during the three and six months ended June 30, 2021, and $0.1 million during the three and six months ended June 30, 2020. Stock Options The fair values of stock options granted during the periods presented are measured on the date of grant using the Black-Scholes option pricing model, with the assumptions noted in the table below. Expected volatility for the Company’s common stock was determined based on an average of the historical volatility of a peer group of publicly traded companies that are similar to the Company. The expected term of options granted to employees was calculated using the simplified method, which represents the average of the contractual term of the option and the weighted-average vesting period of the option. The Company uses the simplified method because it does not have sufficient historical option exercise data to provide a reasonable basis upon which to estimate expected term. The contractual life of the options was used for the expected term of options granted to non-employees. The assumed dividend yield is based upon the Company’s expectation of not paying dividends in the foreseeable future. The risk-free rate is based on the U.S. Treasury yield curve in effect at the time of grant for periods commensurate with the expected term of the award. The Company recognizes forfeitures as they occur. The assumptions used in the Black-Scholes option pricing model are as follows: Three months ended June 30, Six months ended June 30, 2021 2020 2021 2020 Expected volatility 64.6% - 66.9% 64.8% — 65.4% 64.6% - 67.1% 63.2% — 65.4% Weighted-average risk-free interest rate 0.90% - 1.20% 0.33% — 0.54% 0.50% - 1.20% 0.33% — 1.73% Expected dividend yield — % — % — % — % Expected term (in years) 5.5 - 6.25 5.5 - 6.25 5.5 - 6.25 5.5 - 6.25 Underlying common stock fair value $6.53 - $9.62 $12.66 — $18.04 $6.53 - $13.91 $12.66 — $21.75 The following table summarizes the Company’s stock option activity for the six months ended June 30, 2021: Number of Options Weighted- Average Exercise Price per Share Weighted- Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value (in thousands) Outstanding at January 1, 2021 5,840,824 $ 15.18 7.8 $ 12,278 Granted 1,902,700 $ 12.84 Exercised (59,348 ) $ 2.30 Cancelled/Forfeited (158,374 ) $ 18.62 Outstanding at June 30, 2021 7,525,802 $ 14.62 8.0 $ 5,300 Vested and expected to vest at June 30, 2021 7,525,802 $ 14.62 8.0 $ 5,300 Exercisable at June 30, 2021 3,716,324 $ 13.21 7.0 $ 5,131 The total intrinsic value of options exercised during the six months ended June 30, 2021 and 2020 was $0.6 million and $0.5 million, respectively. The weighted-average grant date fair value per share of options granted during the six months ended June 30, 2021 and 2020 was $7.77 and $11.22, respectively. Stock options granted pursuant to the 2015 Plan permit option holders to elect to exercise unvested options in exchange for unvested common stock. Options granted under the 2015 Plan that are exercised prior to vesting will continue to vest according to the respective option agreement, and such unvested shares are subject to repurchase by the Company at the optionee’s original exercise price in the event the optionee’s service with the Company voluntarily or involuntarily terminates. A summary of the Company’s unvested common stock from early exercises that is subject to repurchase by the Company is as follows: Shares Unvested shares - January 1, 2021 3,091 Vested (2,022 ) Issued — Repurchased — Unvested shares - June 30, 2021 1,069 As of June 30, 2021 and December 31, 2020, 1,069 shares and 3,091 shares, respectively, remained subject to a repurchase right by the Company, with a related liability included in accrued expenses and other liabilities in the condensed consolidated balance sheets of less than $0.1 million as of June 30, 2021 and December 31, 2020. Restricted Stock Units The fair values of restricted stock units (“RSUs”) are based on the fair market value of the Company’s common stock on the date of grant. Each RSU represents a contingent right to receive one share of the Company’s common stock upon vesting. In general, RSUs vest annually in three equal installments on January 1 st Number of Restricted Stock Units Weighted- Average Grant Date Fair Value Outstanding at January 1, 2021 — $ — Granted 277,400 13.85 Vested — — Forfeited (4,050 ) 13.91 Outstanding at June 30, 2021 273,350 $ 13.85 Stock-based Compensation Expense The Company recognizes compensation expense for awards to employees based on the grant date fair value of stock-based awards on a straight-line basis over the period during which an award holder provides service in exchange for the award, which is generally the vesting period. The Company recorded stock-based compensation expense related to stock options, shares purchased under the 2018 ESPP and restricted stock units as follows: Three months ended June 30, Six months ended June 30, 2021 2020 2021 2020 (in thousands) Research and development $ 2,303 $ 1,754 $ 4,159 $ 3,068 General and administrative 2,170 1,698 4,197 3,268 $ 4,473 $ 3,452 $ 8,356 $ 6,336 As of June 30, 2021, there was $38.4 million of unrecognized compensation expense related to unvested employee and non-employee share-based compensation arrangements granted under the Plans. The unrecognized compensation expense is estimated to be recognized over a period of 2.6 years at June 30, 2021.

Net Loss Per Share

Net Loss Per Share	6 Months Ended
	Jun. 30, 2021
Earnings Per Share [Abstract]
NET LOSS PER SHARE	9. NET LOSS PER SHARE The Company’s potential dilutive securities, which include unvested common stock from the early-exercise of stock options, outstanding common stock options and outstanding RSUs, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at June 30, 2021 and 2020, from the computation of diluted net loss per share attributable to common stockholders because including them would have had an anti-dilutive effect: As of June 30, 2021 2020 Unvested common stock from early exercise of options 1,069 5,113 Stock options to purchase common stock 7,525,802 5,695,899 Restricted Stock Units 273,350 — Total 7,800,221 5,701,012

Collaboration and License Agree

Collaboration and License Agreement	6 Months Ended
	Jun. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
COLLABORATION AND LICENSE AGREEMENT	10. COLLABORATION AND LICENSE AGREEMENT Termination of the Novartis Collaboration Agreement In November 2017, the Company entered into a collaboration and license agreement with Novartis (as amended, the “Collaboration Agreement”) for the research, development, manufacturing and commercialization of products using the Company’s gene editing technology for the treatment of certain ophthalmic targets and sickle cell disease. In February 2019, Novartis elected to discontinue the sickle cell disease program under the Collaboration Agreement effective in August 2019. Under the terms of the Collaboration Agreement, the Company granted Novartis a research license, a development and commercialization license and a manufacturing license, under certain of its intellectual property rights to research, develop, manufacture and commercialize ophthalmic targets. The Collaboration Agreement included a research term, during which the Company and Novartis were collaborating to perform research and preclinical development work, as well development and commercialization terms, during which the Company would be responsible for manufacturing all product candidates and products. The Company and Novartis were also participating on a joint steering committee and a joint manufacturing subcommittee. The Company had determined that the (1) research, development and commercialization and manufacturing licenses, (2) the research activities performed by the Company, (3) service on the joint committees and (4) manufacturing during the research and development terms represented a single performance obligation under the Collaboration Agreement. On February 26, 2021, Homology received notice from Novartis that they had elected to terminate the Collaboration Agreement with the Company with respect to the remaining ophthalmic target under the Collaboration Agreement and as a result, the Company regained worldwide exclusive rights to this target. Accordingly, the notice served as notice of Novartis’ termination of the Collaboration Agreement in its entirety, which will be effective on August 26, 2021. Under the terms of the Collaboration Agreement, Novartis is obligated to continue to reimburse the Company for certain research and development costs through August 26, 2021. Upon effectiveness of the termination, such reimbursements will cease. The delivery of the termination notice caused a change in the estimate of total costs to satisfy the single performance obligation under the Collaboration Agreement. The cumulative effect of revisions to the total estimated costs to complete the Company’s single performance obligation was recorded in the current period when the changes were identified and amounts could be reasonably estimated. As such, the Company recognized a cumulative effect adjustment of approximately $26.9 million in collaboration revenue in the six months ended June 30, 2021. Revenue Recognition The Company recognizes revenue pursuant to the Collaboration Agreement over time using a cost-to-cost method, which it believes best depicts the transfer of control to the customer. Under the cost-to-cost method, the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the identified performance obligation. Under this method, revenue is recorded over the performance period as a percentage of the estimated transaction price based on the extent of progress towards completion. As of June 30, 2021, the aggregate amount of the transaction price related to the unsatisfied portion of the performance obligation is $0.8 million, and the Company will recognize this revenue as the remaining research activities are performed, which will conclude on August 26, 2021. During the three and six months ended June 30, 2021, the Company recognized revenue under the Collaboration Agreement of $1.4 million and $29.9 million, respectively, of which $1.2 million and $29.4 million was included in deferred revenue at the beginning of each such period. During the three and six months ended June 30, 2020, the Company recognized revenue under the Collaboration Agreement of $0.6 million and $1.2 million, respectively, of which $0.2 million and $0.4 million was included in deferred revenue at the beginning of each period. As of June 30, 2021 and December 31, 2020, there was approximately $0.8 million and $30.2 million of deferred revenue related to the Collaboration Agreement, respectively. In addition, as of June 30, 2021 and December 31, 2020, the Company has recorded accounts receivable of $0.2 million and $0.4 million, respectively, related to reimbursable research and development costs under the Collaboration Agreement, which are included in prepaid expenses and other current assets on the condensed consolidated balance sheets as of each such date.

Pfizer Stock Purchase Agreement

Pfizer Stock Purchase Agreement	6 Months Ended
	Jun. 30, 2021
Pfizer Inc.
PFIZER STOCK PURCHASE AGREEMENT	11. PFIZER STOCK PURCHASE AGREEMENT 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. The shares of common stock sold to Pfizer are subject to a one-year lock-up from closing, during which time Pfizer is prohibited from selling or otherwise disposing of such shares. Under the Stock Purchase Agreement, Pfizer was granted an exclusive right of first refusal (the “ROFR”) for a 30-month period (the “ROFR Period”) beginning on the date of the closing of the Private Placement (collectively, the “ROFR Provision”), to negotiate a potential collaboration on the development and commercialization of HMI-102 and HMI-103. HMI-102 and HMI-103 The Company recorded the issuance of common stock at its estimated fair value of $52.0 million, which reflects a discount for the lack of marketability of the shares. The remaining $8.0 million of aggregate purchase price was allocated to the other elements of the Stock Purchase Agreement, which represent a contract with a customer. The Company concluded that the Information Committee represents the only performance obligation under the contract. The ROFR does not provide Pfizer with a material right and is therefore not a performance obligation. As such, the Company allocated the $8.0 million to the Information Committee obligation. The Company recognizes revenue over time as the measure of progress, which it believes best depicts the transfer of control to Pfizer. The Information Committee will meet regularly over the ROFR Period to share information which results in recognition of the transaction price over the 30-month ROFR Period. During the three and six months ended June 30, 2021, the Company recognized collaboration revenue of $0.8 million and $1.6 million, respectively, and as of June 30, 2021, there was approximately $ 6.0

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation	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 June 30, 2021, and consolidated results of operations for the three and six months ended June 30, 2021 and 2020, and cash flows for the six months ended June 30, 2021 and 2020. Such adjustments are of a normal and recurring nature. The results of operations for the three and six months ended June 30, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021.
Principles of Consolidation	Principles of Consolidation —The Company’s condensed consolidated financial statements include the accounts of the Company and its subsidiary, Homology Medicines Securities Corporation, a wholly owned Massachusetts corporation, for the sole purpose of buying, selling, and holding securities on the Company’s behalf. All intercompany balances and transactions have been eliminated in the condensed consolidated financial statements.
Use of Estimates	Use of Estimates —The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenue, and expenses, and the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases its estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, revenue recognition, accrued research and development expenses and useful lives assigned to property and equipment. The Company assesses estimates on an ongoing basis; however, actual results could materially differ from those estimates.
Comprehensive Income (Loss)	Comprehensive Income (Loss) —Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. The Company’s only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments.
Cash and Cash Equivalents and Restricted Cash	Cash and Cash Equivalents and Restricted Cash —Cash and cash equivalents consist of standard checking accounts, money market accounts and certain investments. The Company considers all highly liquid investments with original or remaining maturities at the time of purchase of 90 days or less to be cash equivalents. Restricted cash consists of cash serving as collateral for letters of credit issued for security deposits for the Company’s facility leases in Bedford, Massachusetts. The following table provides a reconciliation of cash, cash equivalents and restricted cash to amounts shown in the condensed consolidated statements of cash flows: June 30, 2021 2020 (in thousands) Cash and cash equivalents $ 116,313 $ 167,636 Restricted cash 1,274 1,274 Total cash, cash equivalents and restricted cash $ 117,587 $ 168,910
Short-Term Investments	Short-Term Investments —Short-term investments represent holdings of available-for-sale marketable securities in accordance with the Company’s investment policy and cash management strategy. Short-term investments have maturities of greater than 90 days at the time of purchase and mature within one year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses reported within accumulated other comprehensive income as a separate component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the underlying security. Such amortization and accretion, together with interest on securities, are included in interest income in the Company’s condensed consolidated statements of operations. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other income.
Offering Costs	Offering Costs —The Company capitalizes incremental legal, professional accounting and other third-party fees that are directly associated with equity financings as other current assets until the transactions are completed. After equity financings are complete, these costs are recorded in stockholders’ equity as a reduction of additional paid-in capital generated as a result of the offering.
Leases	Leases —The Company determines if an arrangement is a lease at contract inception. The Company’s contracts are determined to contain a lease when all of the following criteria based on the specific circumstances of the arrangement are met: (1) there is an identified asset for which there are no substantive substitution rights; (2) the Company has the right to obtain substantially all of the economic benefits from the identified asset; and (3) the Company has the right to direct the use of the identified asset. At the commencement date, operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of future lease payments over the expected lease term. The Company’s lease agreements do not provide an implicit rate. As a result, the Company utilizes an estimated incremental borrowing rate to discount lease payments, which is based on the rate of interest the Company would have to pay to borrow a similar amount on a collateralized basis over a similar term. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or lease incentives received. Operating lease cost is recognized over the expected term on a straight-line basis. The expected lease term includes noncancelable lease periods and, when applicable, periods covered by an option to extend the lease if the Company is reasonably certain to exercise that option, as well as periods covered by an option to terminate the lease if the Company is reasonably certain not to exercise that option. Variable lease cost is recognized as incurred. The Company acts as sublessor related to a sublease of the Company's former headquarters. Fixed sublease payments received are recognized on a straight-line basis over the sublease term and recorded as a reduction to lease cost. Right-of-use assets are periodically evaluated for impairment.
Research and Development Costs	Research and Development Costs —Research and development costs are charged to expense as incurred. Research and development expense consists of expenses incurred in performing research and development activities, including salaries and benefits, materials and supplies, preclinical and clinical expenses, stock-based compensation expense, depreciation of equipment, contract services, and other outside expenses. Costs for certain development activities are recognized based on an evaluation of the progress to completion of specific tasks using information provided to the Company by its vendors on their actual costs incurred. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid expense or accrued research and development expense.
Revenue Recognition	Revenue Recognition— Revenue is recognized in accordance with FASB Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”). On January 1, 2019, the Company adopted ASC 606 using the full retrospective transition method. Under ASC 606, the Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations , including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in the Company’s arrangements would likely consist of a license, rights to the Company’s intellectual property or research, development and manufacturing services. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of consideration to which the Company expects to be entitled to. The Company utilizes either the most likely amount method or expected value method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration that is included in the transaction price may be constrained and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts may include development and regulatory milestone payments that are assessed under the most likely amount method and constrained until it is probable that a significant revenue reversal would not occur. Milestone payments that are not within the Company’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such development and regulatory milestones and any related constraint, and if necessary, adjust its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from the Company’s collaboration arrangement. The Company allocates the transaction price based on the estimated standalone selling price of each performance obligation. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress for its over-time arrangements at each reporting period and, if necessary, updates the measure of progress and revenue recognized.
Net Loss per Share	Net Loss per Share— Basic net loss per share is computed by dividing net loss by the weighted-average number of common shares outstanding during the period. Diluted net loss per share is computed using the weighted-average number of common shares outstanding during the period and, if dilutive, the weighted-average number of potential shares of common stock. The weighted-average number of common shares included in the computation of diluted net loss gives effect to all potentially dilutive common equivalent shares, including outstanding stock options, restricted stock units and unvested shares of common stock. Common stock equivalent shares are excluded from the computation of diluted net loss per share if their effect is antidilutive. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is generally the same as basic net loss per share attributable to common stockholders , since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements —The Jumpstart Our Business Startups Act of 2012 permits an emerging growth company to take advantage of an extended transition period to comply with new or revised accounting standards applicable to public companies until those standards would otherwise apply to private companies. As an emerging growth company, the Company has elected to take advantage of this extended transition period. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In October 2020, the FASB issued ASU 2020-10, Codification Improvements All other new accounting pronouncements issued, but not yet effective or adopted have been deemed to be not relevant to the Company and, accordingly, are not expected to have a material impact once adopted.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash to amounts shown in the condensed consolidated statements of cash flows: June 30, 2021 2020 (in thousands) Cash and cash equivalents $ 116,313 $ 167,636 Restricted cash 1,274 1,274 Total cash, cash equivalents and restricted cash $ 117,587 $ 168,910

Short-Term Investments (Tables)

Short-Term Investments (Tables)	6 Months Ended
	Jun. 30, 2021
Investments Disclosure [Abstract]
Summary of Short Term Investments	The following table summarizes the Company’s short-term investments as of June 30, 2021: As of June 30, 2021 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Commercial paper $ 61,944 $ — $ — $ 61,944 Corporate debt securities 35,034 4 (2 ) 35,036 Total $ 96,978 $ 4 $ (2 ) $ 96,980

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2021
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on Recurring Basis	Assets measured at fair value on a recurring basis were as follows: Description June 30, 2021 Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Money market mutual funds $ 116,063 $ 116,063 $ — $ — Total cash equivalents $ 116,063 $ 116,063 $ — $ — Short-term investments: Commercial paper $ 61,944 $ — $ 61,944 $ — Corporate debt securities 35,036 — 35,036 — Total short-term investments $ 96,980 $ — $ 96,980 $ — Total financial assets $ 213,043 $ 116,063 $ 96,980 $ — Description December 31, 2020 Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Money market mutual funds $ 217,181 $ 217,181 $ — $ — Total financial assets $ 217,181 $ 217,181 $ — $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	6 Months Ended
	Jun. 30, 2021
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consists of the following: As of June 30, 2021 December 31, 2020 (in thousands) Laboratory equipment $ 13,752 $ 12,703 Manufacturing equipment 7,877 7,754 Computers and purchased software 1,597 1,596 Furniture and fixtures 1,625 1,363 Leasehold improvements 29,961 29,961 Assets not yet in service 63 — Property and equipment, at cost 54,875 53,377 Less: accumulated depreciation and amortization (20,496 ) (16,375 ) Property and equipment, net $ 34,379 $ 37,002

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	6 Months Ended
	Jun. 30, 2021
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consist of the following: As of June 30, 2021 December 31, 2020 (in thousands) Accrued compensation and benefits $ 5,254 $ 6,902 Accrued research and development expenses 1,858 2,393 Accrued professional fees 397 291 Accrued other 180 217 Total accrued expenses and other liabilities $ 7,689 $ 9,803

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Schedule of Operating Lease Costs, Variable Lease Costs and Sub Lease Income	The following table summarizes operating lease costs, variable lease costs and sublease income: Six months ended June 30, 2021 2020 (in thousands) Operating lease costs $ 1,246 $ 1,246 Variable lease costs 1,032 1,029 Sublease income (470 ) (457 ) Net lease cost $ 1,808 $ 1,818
Schedule of Operating Lease Liabilities and Minimum Lease Payments	The maturities of our operating lease liabilities and minimum lease payments as of June 30, 2021 were as follows: For the Years Ending December 31, Amount (in thousands) 2021 1,892 2022 2,987 2023 3,077 2024 3,169 Thereafter 7,197 Total undiscounted lease payments $ 18,322 Less: imputed interest (4,133 ) Present value of operating lease liabilities $ 14,189
Schedule of Lease Term and Discount Rate	The following table summarizes the lease term and discount rate as of June 30, 2021: As of June 30, 2021 Weighted-average remaining lease term (years) Operating leases 5.5 Weighted-average discount rate Operating leases 10.0 %
Schedule of Operating Lease Liabilities	The following table summarizes cash paid for amounts included in the measurement of the Company’s operating lease liabilities. Six months ended June 30, 2021 2020 (in thousands) Cash paid for amounts included in the measurement of lease liabilities $ 1,930 $ 1,886

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model	The assumptions used in the Black-Scholes option pricing model are as follows: Three months ended June 30, Six months ended June 30, 2021 2020 2021 2020 Expected volatility 64.6% - 66.9% 64.8% — 65.4% 64.6% - 67.1% 63.2% — 65.4% Weighted-average risk-free interest rate 0.90% - 1.20% 0.33% — 0.54% 0.50% - 1.20% 0.33% — 1.73% Expected dividend yield — % — % — % — % Expected term (in years) 5.5 - 6.25 5.5 - 6.25 5.5 - 6.25 5.5 - 6.25 Underlying common stock fair value $6.53 - $9.62 $12.66 — $18.04 $6.53 - $13.91 $12.66 — $21.75
Summary of Option Activity under Plans	The following table summarizes the Company’s stock option activity for the six months ended June 30, 2021: Number of Options Weighted- Average Exercise Price per Share Weighted- Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value (in thousands) Outstanding at January 1, 2021 5,840,824 $ 15.18 7.8 $ 12,278 Granted 1,902,700 $ 12.84 Exercised (59,348 ) $ 2.30 Cancelled/Forfeited (158,374 ) $ 18.62 Outstanding at June 30, 2021 7,525,802 $ 14.62 8.0 $ 5,300 Vested and expected to vest at June 30, 2021 7,525,802 $ 14.62 8.0 $ 5,300 Exercisable at June 30, 2021 3,716,324 $ 13.21 7.0 $ 5,131
Summary of Unvested Common Stock from Early Exercises Subject to Repurchase	A summary of the Company’s unvested common stock from early exercises that is subject to repurchase by the Company is as follows: Shares Unvested shares - January 1, 2021 3,091 Vested (2,022 ) Issued — Repurchased — Unvested shares - June 30, 2021 1,069
Summary of Company's RSU Activity	The following table summarizes the Company’s RSU activity for the Number of Restricted Stock Units Weighted- Average Grant Date Fair Value Outstanding at January 1, 2021 — $ — Granted 277,400 13.85 Vested — — Forfeited (4,050 ) 13.91 Outstanding at June 30, 2021 273,350 $ 13.85
Schedule of Stock-Based Compensation Expense	The Company recorded stock-based compensation expense related to stock options, shares purchased under the 2018 ESPP and restricted stock units as follows: Three months ended June 30, Six months ended June 30, 2021 2020 2021 2020 (in thousands) Research and development $ 2,303 $ 1,754 $ 4,159 $ 3,068 General and administrative 2,170 1,698 4,197 3,268 $ 4,473 $ 3,452 $ 8,356 $ 6,336

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	6 Months Ended
	Jun. 30, 2021
Earnings Per Share [Abstract]
Schedule of Anti-dilutive Effect Excluded from Computation of Diluted Net Loss Per Share Attributable to Common Stockholders	The Company excluded the following potential common shares, presented based on amounts outstanding at June 30, 2021 and 2020, from the computation of diluted net loss per share attributable to common stockholders because including them would have had an anti-dilutive effect: As of June 30, 2021 2020 Unvested common stock from early exercise of options 1,069 5,113 Stock options to purchase common stock 7,525,802 5,695,899 Restricted Stock Units 273,350 — Total 7,800,221 5,701,012

Nature of Business and Basis _2

Nature of Business and Basis of Presentation - Additional Information (Details) - USD ($)	Apr. 06, 2021	Nov. 09, 2020	Mar. 12, 2020	Apr. 12, 2019	Apr. 02, 2018	Nov. 09, 2020	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Conversion of convertible preferred stock into common stock upon initial public offering, Shares					24,168,656
Net proceed from issuance of common stock											$ 49,744,000
Net loss							$ 30,500,000	$ 1,073,000	$ 35,340,000	$ 35,331,000	31,573,000	$ 70,671,000
Accumulated deficit							$ 359,941,000				$ 359,941,000		$ 328,368,000
Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings			$ 148,400
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Issuance of common stock, net of discounts and issuance costs, Shares					10,350,000
Common stock at a public offering price					$ 16
Net proceeds from initial public offering after underwriting discounts and commissions and offering expenses					$ 150,800,000
Follow-on Offering [Member] | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Issuance of common stock, net of discounts and issuance costs, Shares	6,596,306			5,555,556			6,596,306
Shares issued price per share	$ 7.58			$ 22.50
Net proceeds after deducting underwriting discounts and commissions and offering expenses	$ 49,700			$ 116,900,000
Underwriters Option | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Issuance of common stock, net of discounts and issuance costs, Shares	989,445			833,333
Shares issued price per share	$ 7.58			$ 22.50
Net proceeds after deducting underwriting discounts and commissions and offering expenses				$ 17,600,000
Private Placement
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Issuance of common stock, net of discounts and issuance costs, Shares		5,000,000				5,000,000
Shares issued price per share		$ 12				$ 12
Net proceed from issuance of common stock		$ 60,000,000				$ 60,000,000
ATM
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Issuance of common stock, net of discounts and issuance costs, Shares											114,914
Shares issued price per share							$ 14				$ 14
Net proceed from issuance of common stock											$ 1,500,000
Proceeds through future financings											$ 148,400,000
Commission Fee											3.00%
Payments Of Stock Issuance Costs											$ 100,000
ATM | Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings			$ 150,000,000
ATM | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Issuance of common stock, net of discounts and issuance costs, Shares								114,914

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Accounting Policies [Abstract]
Cash and cash equivalents	$ 116,313	$ 217,431	$ 167,636
Restricted cash	1,274	$ 1,274	1,274
Total cash, cash equivalents and restricted cash	$ 117,587		$ 168,910

Short-Term Investments - Summar

Short-Term Investments - Summary of Short Term Investments (Details) $ in Thousands	Jun. 30, 2021USD ($)
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	$ 96,978
Cash equivalents and short-term investments, Unrealized Gains	4
Cash equivalents and short-term investments, Unrealized Losses	(2)
Cash equivalents and short-term investments, Fair Value	96,980
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	61,944
Cash equivalents and short-term investments, Fair Value	61,944
Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	35,034
Cash equivalents and short-term investments, Unrealized Gains	4
Cash equivalents and short-term investments, Unrealized Losses	(2)
Cash equivalents and short-term investments, Fair Value	$ 35,036

Short-Term Investments - Additi

Short-Term Investments - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Cash Cash Equivalents And Available For Sale Securities [Abstract]
Short-term investments	$ 96,980,000		$ 96,980,000		$ 0
Realized gains and losses on available-for-sale securities	$ 0	$ 0	$ 0	$ 0
Contractual maturity date of investments	less than one year

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	$ 96,980,000	$ 0
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	116,063,000
Short-term investments, fair value	96,980,000
Financial assets, fair value	213,043,000	217,181,000
Fair Value, Measurements, Recurring | Money Market Mutual Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	116,063,000	217,181,000
Fair Value, Measurements, Recurring | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	61,944,000
Fair Value, Measurements, Recurring | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	35,036,000
Fair Value, Measurements, Recurring | Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	116,063,000
Financial assets, fair value	116,063,000	217,181,000
Fair Value, Measurements, Recurring | Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) | Money Market Mutual Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	116,063,000	$ 217,181,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	96,980,000
Financial assets, fair value	96,980,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	61,944,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	$ 35,036,000

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Fair Value Disclosures [Abstract]
Transfers between fair value measure levels	$ 0	$ 0	$ 0	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property and equipment, at cost	$ 54,875	$ 53,377
Less: accumulated depreciation and amortization	(20,496)	(16,375)
Property and equipment, net	34,379	37,002
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, at cost	13,752	12,703
Manufacturing Equipment
Property Plant And Equipment [Line Items]
Property and equipment, at cost	7,877	7,754
Computers and Purchased Software
Property Plant And Equipment [Line Items]
Property and equipment, at cost	1,597	1,596
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, at cost	1,625	1,363
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, at cost	29,961	$ 29,961
Assets Not Yet in Service
Property Plant And Equipment [Line Items]
Property and equipment, at cost	$ 63

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Property Plant And Equipment [Line Items]
Depreciation		$ 2,000,000	$ 4,181,000	$ 3,899,000
Disposal of property and equipment		$ 0		$ 0
Maximum
Property Plant And Equipment [Line Items]
Depreciation	$ 2,100,000		4,200,000
Disposal of property and equipment	$ 100,000		$ 100,000

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Accrued compensation and benefits	$ 5,254	$ 6,902
Accrued research and development expenses	1,858	2,393
Accrued professional fees	397	291
Accrued other	180	217
Total accrued expenses and other liabilities	$ 7,689	$ 9,803

Commitment and Contingencies -

Commitment and Contingencies - Additional Information (Details)	1 Months Ended
	Dec. 31, 2017ft²Phase$ / ft²	Sep. 30, 2016	Jun. 30, 2021USD ($)	Dec. 31, 2020USD ($)
Letters Of Credit And Secured By Restricted Cash
Commitment And Contingencies [Line Items]
Security deposit | $			$ 1,300,000	$ 1,300,000
Maximum
Commitment And Contingencies [Line Items]
Tenant improvement allowance | $			10,900,000
Bedford, Massachusetts
Commitment And Contingencies [Line Items]
Operating lease beginning year and month		2016-11
Operating lease expiration year and month	2027-02	2021-10
Operating lease agreements additional term		3 years
Sublease aggregate base rent obligation | $			$ 2,700,000
Office space leased | ft²	67,000
Lessee, operating lease, lease not yet commenced, renewal term	5 years
Number of phases | Phase	2
Initial annual base rent per square foot | $ / ft²	39.50
Percentage increase in initial annual base rent per square foot.	3.00%
Bedford, Massachusetts | Phase One
Commitment And Contingencies [Line Items]
Office space leased | ft²	46,000
Rent due date	2018-09
Bedford, Massachusetts | Phase Two
Commitment And Contingencies [Line Items]
Office space leased | ft²	21,000
Rent due date	2019-03

Commitment and Contingencies _2

Commitment and Contingencies - Schedule of Operating Lease Costs, Variable Lease Costs and Sub Lease Income (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Operating lease costs	$ 1,246	$ 1,246
Variable lease costs	1,032	1,029
Sublease income	(470)	(457)
Net lease cost	$ 1,808	$ 1,818

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Operating Lease Liabilities and Minimum Lease Payments (Details) $ in Thousands	Jun. 30, 2021USD ($)
Commitments And Contingencies Disclosure [Abstract]
2021	$ 1,892
2022	2,987
2023	3,077
2024	3,169
Thereafter	7,197
Total undiscounted lease payments	18,322
Less: imputed interest	(4,133)
Present value of operating lease liabilities	$ 14,189

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Lease Term and Discount Rate (Details)	Jun. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Weighted-average remaining lease term (years), Operating leases	5 years 6 months
Weighted-average discount rate, Operating leases	10.00%

Commitments and Contingencies_3

Commitments and Contingencies - Summary of Cash Paid for Company's Operating Lease Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Cash paid for amounts included in the measurement of lease liabilities	$ 1,930	$ 1,886

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2021	Mar. 31, 2018	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate proceeds from shares issued under the plan					$ 446	$ 537
Stock-based compensation			$ 4,473	$ 3,452	8,356	6,336
Total intrinsic value of options exercised					$ 600	$ 500
Weighted-average grant date fair value per share for options granted					$ 7.77	$ 11.22
2015 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting period					4 years
Stock options expiration period					10 years
Number of additional shares available for future grant			0		0
2018 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance		3,186,205
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock	4.00%	4.00%
Maximum shares of common stock may be issued		20,887,347
Number of additional shares added to the plan	2,010,623
Number of shares outstanding available for future grant			2,390,494		2,390,494
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance		353,980
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock	1.00%	1.00%
Number of additional shares added to the plan	502,655
Number of shares outstanding available for future grant			1,493,513		1,493,513
Maximum shares allowed to be issued under ESPP		4,778,738
Purchase of common stock through payroll deductions expressed in percentage of fair market value				85.00%
Common stock offering period				6 months
Number of shares issued to the plan					48,792	39,471
Aggregate proceeds from shares issued under the plan					$ 400	$ 500
Stock-based compensation			$ 100	$ 100	100	$ 100
2015 and 2018 Stock Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense			$ 38,400		$ 38,400
Unrecognized compensation expense estimated to be recognized over period					2 years 7 months 6 days

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model (Details) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	64.60%	64.80%	64.60%	63.20%
Expected volatility, maximum	66.90%	65.40%	67.10%	65.40%
Weighted-average risk-free interest rate, minimum	0.90%	0.33%	0.50%	0.33%
Weighted-average risk-free interest rate, maximum	1.20%	0.54%	1.20%	1.73%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months	6 years 3 months
Underlying common stock fair value	$ 9.62	$ 18.04	$ 13.91	$ 21.75
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 6 months	5 years 6 months	5 years 6 months	5 years 6 months
Underlying common stock fair value	$ 6.53	$ 12.66	$ 6.53	$ 12.66

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Option Activity under Plans (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding at Beginning Balance | shares	5,840,824
Number of Options, Granted | shares	1,902,700
Number of Options, Exercised | shares	(59,348)
Number of Options, Cancelled/Forfeited | shares	(158,374)
Number of Options, Outstanding at Ending Balance | shares	7,525,802	5,840,824
Number of Options, Vested and Expected to vest at March 31, 2021 | shares	7,525,802
Number of Options, Exercisable at March 31, 2021 | shares	3,716,324
Weighted-Average Exercise Price per Share, Outstanding at Beginning Balance | $ / shares	$ 15.18
Weighted-Average Exercise Price per Share, Granted | $ / shares	12.84
Weighted-Average Exercise Price per Share, Exercised | $ / shares	2.30
Weighted-Average Exercise Price per Share, Cancelled/Forfeited | $ / shares	18.62
Weighted-Average Exercise Price per Share, Outstanding at Ending Balance | $ / shares	14.62	$ 15.18
Weighted-Average Exercise Price per Share, Vested and Expected to vest at March 31, 2021 | $ / shares	14.62
Weighted-Average Exercise Price Per Share, Exercisable at March 31, 2021 | $ / shares	$ 13.21
Weighted Average Remaining Contractual Term, Outstanding	8 years	7 years 9 months 18 days
Weighted Average Remaining Contractual Term, Vested and Expected to vest at March 31, 2021	8 years
Weighted Average Remaining Contractual Term, Exercisable at March 31, 2021	7 years
Aggregate Intrinsic Value, Outstanding Ending Balance | $	$ 5,300	$ 12,278
Aggregate Intrinsic Value, Vested and Expected to vest at March 31, 2021 | $	5,300
Aggregate Intrinsic Value, Exercisable at March 31, 2021 | $	$ 5,131

Stock Incentive Plans - Summa_2

Stock Incentive Plans - Summary of Unvested Common Stock from Early Exercises Subject to Repurchase (Details)	6 Months Ended
	Jun. 30, 2021shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unvested shares, Beginning Balance	3,091
Vested	(2,022)
Unvested shares, Ending Balance	1,069

Stock Incentive Plans - Summa_3

Stock Incentive Plans - Summary of RSU Activity (Details) - $ / shares	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Outstanding at Beginning Balance	5,840,824
Number of Options, Granted	1,902,700
Number of Options, Outstanding at Ending Balance	7,525,802
Weighted-average grant date fair value per share for options granted	$ 7.77	$ 11.22
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Granted	277,400
Number of Restricted Stock Units, Forfeited	(4,050)
Number of Options, Outstanding at Ending Balance	273,350
Weighted-average grant date fair value per share for options granted	$ 13.85
Weighted-Average Grant Date Fair Value, Forfeited	13.91
Weighted-Average Grant Date Fair Value, Outstanding	$ 13.85

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	$ 4,473	$ 3,452	$ 8,356	$ 6,336
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	2,303	1,754	4,159	3,068
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	$ 2,170	$ 1,698	$ 4,197	$ 3,268

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Anti-dilutive Effect Excluded from Computation of Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - shares	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	7,800,221	5,701,012
Unvested Common Stock from Early Exercise of Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	1,069	5,113
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	7,525,802	5,695,899
Restricted Stock Units (RSUs)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	273,350

Collaboration and License Agr_2

Collaboration and License Agreement - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Nov. 30, 2017	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Apr. 01, 2021	Jan. 01, 2021	Dec. 31, 2020	Apr. 01, 2020	Jan. 01, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement revenue recognized		$ 2,187	$ 567	$ 31,492	$ 1,155
Novartis
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration Revenue	$ 26,900
Novartis | Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement revenue recognized		1,400	$ 600	29,900	$ 1,200
Deferred revenue		800		800		$ 1,200	$ 29,400	$ 30,200	$ 200	$ 400
Novartis | Collaborative Arrangement | Reimbursable Research and Development Costs
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable		$ 200		$ 200				$ 400

Collaboration and License Agr_3

Collaboration and License Agreement - Additional Information (Details1) - Novartis $ in Millions	Jun. 30, 2021USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2017-11-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining performance obligation, unsatisfied portion	$ 0.8
Remaining performance obligation, expected timing of satisfaction, period	10 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2027-04-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining performance obligation, unsatisfied portion	$ 0.8
Remaining performance obligation, expected timing of satisfaction, period	10 years

Pfizer Stock Purchase Agreeme_2

Pfizer Stock Purchase Agreement - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 09, 2020	Nov. 09, 2020	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net proceed from issuance of common stock					$ 49,744
Collaboration agreement revenue recognized			$ 2,187	$ 567	31,492	$ 1,155
Pfizer Inc.
Common stock estimated fair value	$ 52,000
Stock purchase agreement remaining allocated value	8,000	$ 8,000
Allocated Information Committee obligation	$ 8,000	$ 8,000
Pfizer Inc. | Collaborative Arrangement
Collaboration agreement revenue recognized			800		1,600
Deferred revenue			$ 6,000		$ 6,000
Private Placement
Issuance of common stock, net of discounts and issuance costs, Shares	5,000,000	5,000,000
Shares issued price per share	$ 12	$ 12
Net proceed from issuance of common stock	$ 60,000	$ 60,000
Common stock purchase agreement condition	The shares of common stock sold to Pfizer are subject to a one-year lock-up from closing, during which time Pfizer is prohibited from selling or otherwise disposing of such shares.