Q32 Bio (QTTB) 10-Q 7 May 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 01, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Period Focus	Q1
Document Fiscal Year Focus	2020
Entity Registrant Name	Homology Medicines, Inc.
Entity Central Index Key	0001661998
Trading Symbol	FIXX
Current Fiscal Year End Date	--12-31
Entity Filer Category	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		45,213,368
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 ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 143,372	$ 53,774
Short-term investments	91,557	208,614
Prepaid expenses and other current assets	3,404	4,189
Total current assets	238,333	266,577
Property and equipment, net	42,566	42,716
Right-of-use assets	6,615
Restricted cash	1,274	1,274
Total assets	288,788	310,567
Current liabilities:
Accounts payable	7,398	2,608
Accrued expenses and other liabilities	6,949	7,644
Operating lease liabilities	2,331
Deferred rent		1,313
Deferred revenue	774	809
Total current liabilities	17,452	12,374
Non-current liabilities:
Operating lease liabilities, net of current portion	14,821
Deferred rent, net of current portion		9,544
Deferred revenue, net of current portion	29,974	30,142
Total liabilities	62,247	52,060
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 10,000,000 shares authorized; no shares issued and outstanding
Common stock, $0.0001 par value; 200,000,000 shares authorized; 45,212,190 and 45,138,408 shares issued as of March 31, 2020 and December 31, 2019, respectively; and 45,206,066 and 45,116,742 shares outstanding as of March 31, 2020 and December 31, 2019, respectively	4	4
Additional paid-in capital	461,561	457,994
Accumulated other comprehensive gain (loss)	(19)	183
Accumulated deficit	(235,005)	(199,674)
Total stockholders’ equity	226,541	258,507
Total liabilities and stockholders' equity	$ 288,788	$ 310,567

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
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	45,212,190	45,138,408
Common stock, shares outstanding	45,206,066	45,116,742

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue	$ 588	$ 270
Operating expenses:
Research and development	29,310	20,536
General and administrative	7,770	4,857
Total operating expenses	37,080	25,393
Loss from operations	(36,492)	(25,123)
Other income:
Interest income	1,161	1,271
Total other income	1,161	1,271
Net loss	$ (35,331)	$ (23,852)
Net loss per share-basic and diluted	$ (0.78)	$ (0.64)
Weighted-average common shares outstanding-basic and diluted	45,151,265	37,384,507

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (35,331)	$ (23,852)
Other comprehensive gain (loss):
Change in unrealized gain (loss) on available for sale securities, net	(202)	83
Total other comprehensive gain (loss)	(202)	83
Comprehensive loss	$ (35,533)	$ (23,769)

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ 196,355	$ 3	$ 292,187	$ (77)	$ (95,758)
Beginning balance, Shares at Dec. 31, 2018		37,358,526
Vesting of common stock from option exercises	18		18
Vesting of common stock from option exercises, Shares		31,584
Issuance of common stock from option exercises	21		21
Issuance of common stock from option exercises, Shares		18,876
Issuance of common stock pursuant to employee stock purchase plan	$ 396		396
Issuance of common stock pursuant to employee stock purchase plan, Shares	28,311	28,311
Stock-based compensation	$ 1,243		1,243
Other comprehensive gain (loss)	83			83
Net loss	(23,852)				(23,852)
Ending balance at Mar. 31, 2019	174,264	$ 3	293,865	6	(119,610)
Ending balance, Shares at Mar. 31, 2019		37,437,297
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	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

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (35,331)	$ (23,852)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,907	1,266
Amortization of right-of-use assets	221
Stock-based compensation expense	2,883	1,243
Amortization of premium on short-term investments	(175)	(661)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,087	4,413
Accounts payable	4,841	(85)
Accrued expenses and other liabilities	(535)	(1,404)
Deferred revenue	(203)	(92)
Deferred rent		1,301
Operating lease liabilities	(541)
Net cash used in operating activities	(25,846)	(17,871)
Cash flows from investing activities:
Purchases of short-term investments		(84,467)
Maturities of short-term investments	117,030	114,600
Purchases of property and equipment	(2,033)	(10,466)
Net cash provided by investing activities	114,997	19,667
Cash flows from financing activities:
Proceeds from issuance of common stock pursuant to employee stock purchase plan	537	396
Proceeds from issuance of common stock from option exercises	135	21
Payment of deferred offering costs	225	92
Net cash provided by financing activities	447	325
Net change in cash, cash equivalents and restricted cash	89,598	2,121
Cash, cash equivalents and restricted cash, beginning of period	55,048	39,992
Cash, cash equivalents and restricted cash, end of period	144,646	42,113
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	12	18
Property and equipment additions included in accounts payable	405	5,274
Property and equipment additions included in accrued expenses and other liabilities	23
Unrealized gain on available for sale securities, net	(202)	83
Deferred offering costs included in accounts payable and accrued expenses	$ 77	$ 174

Nature of Business and Basis of

Nature of Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2020
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 biopharmaceutical company dedicated to translating proprietary gene editing and gene therapy 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 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, obtain regulatory approval of its products, 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 March 12, 2020, the Company filed a Registration Statement on Form S-3 (File No. 333-237131) (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”). The Company did not sell any shares of common stock pursuant to the ATM during the three months ended March 31, 2020. At March 31, 2020, there remained $150.0 million of common stock available for sale under the ATM. Simultaneous with the filing of the Shelf, the previous Registration Statement on Form S-3 (File No. 333-230664) filed with the SEC on April 1, 2019, and a prior sales agreement with Cowen providing for the offering, issuance and sale by the Company of up to an aggregate $100.0 million of its common stock from time to time in “at-the-market” offerings, with $76.8 million of remaining capacity, were terminated. To date, the Company has not generated any revenue from product sales and does not expect to generate any revenue from the sale of product in the foreseeable future. Through March 31, 2020, 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). During the three months ended March 31, 2020, the Company incurred a net loss of $35.3 million and as of March 31, 2020, had $235.0 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 existing cash, cash equivalents and short-term investments will enable the Company to continue its operations into the fourth quarter of 2021. 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, 2019, included in the Company's Annual Report on Form 10-K on file with the SEC. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position as of March 31, 2020, and consolidated results of operations for the three months ended March 31, 2020 and 2019, and cash flows for the three months ended March 31, 2020 and 2019. Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2020.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2020
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: March 31, 2020 2019 (in thousands) Cash and cash equivalents $ 143,372 $ 40,839 Restricted cash 1,274 1,274 Total cash, cash equivalents and restricted cash $ 144,646 $ 42,113 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 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. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion, together with interest on securities, are included in interest income on 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. Deferred 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 — In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which eliminated the tests for lease classification under prior U.S. GAAP and requires lessees to recognize right-of-use assets and related lease liabilities on the balance sheet. The FASB subsequently issued the following amendments to ASU 2016-02 that have the same effective date and transition date: ASU No. 2018-01 - Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842 , ASU No. 2018-10 - Codification Improvements to Topic 842 , Leases , ASU No. 2018-11 – Leases (Topic 842): Targeted Improvements , ASU No. 2018-20 – Leases (Topic 842): Narrow-Scope Improvements for Lessors , and ASU No. 2019-01 – Leases (Topic 842): Codification Improvements . The Company adopted these amendments with ASU 2016-02 (collectively, the new leasing standards) effective January 1, 2020. The Company adopted the new leasing standards using the modified retrospective approach, as of January 1, 2020, with no restatement of prior periods or cumulative adjustments to retained earnings. The Company elected the package of practical expedients permitted under the transition guidance within the new standard, which, among other things, allows the Company to carry forward the historical lease classification. In addition, the Company elected the practical expedient not to apply the recognition requirements in the leasing standards to short-term leases (a lease that at commencement date has a lease term of 12 months or less and does not contain a purchase option that it is reasonably certain to exercise) and the practical expedient that permits lessees to make an accounting policy election (by class of underlying asset) to not separate lease components of a contract from non-lease components. 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. 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. Right-of-use assets are periodically evaluated for impairment. The expected lease term for those leases commencing prior to January 1, 2020 did not change with the adoption of the new leasing standards. The expected lease term for leases commencing after the adoption of the new leasing standards 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. As a result of the adoption of the new leasing standards, on January 1, 2020, the Company recorded a right-of-use asset of $6.8 million, operating lease liabilities of $17.7 million and the derecognition of deferred rent of $10.9 million originally accounted for under legacy guidance. The adoption did not have a material impact on the condensed consolidated statement of operations. For additional information on the adoption of the new leasing standards, refer to Note 7. January 1, 2020 (in thousands) Prior to adoption of new leasing standards Adjustment for adoption of new leasing standards As Adjusted Right-of-use assets (1)(2) $ — $ 6,835 $ 6,835 Deferred rent (2) $ 1,313 $ (1,313 ) $ — Deferred rent, net of current portion (2) $ 9,544 $ (9,544 ) $ — Operating lease liabilities (3) $ — $ 2,251 $ 2,251 Operating lease liabilities, net of current portion (3) $ — $ 15,441 $ 15,441 (1) Represents capitalization of operating right-of-use assets (2) Represents reclassification of deferred rent and incentives as a reduction to operating right-of-use assets (3) Represents recognition of operating lease liabilities 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 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 February 2016, the FASB issued new leasing standards to increase transparency and comparability among organization related to their leasing activities. For additional information on the adoption of the new leasing standards, please refer to section titled “Leases” above, and Note 7. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ,

Short-Term Investments

Short-Term Investments	3 Months Ended
	Mar. 31, 2020
Investments Disclosure [Abstract]
SHORT-TERM INVESTMENTS	3. SHORT-TERM INVESTMENTS The Company invests 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. 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 investments, which are included in cash equivalents and short-term investments: As of March 31, 2020 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Asset-backed securities $ 4,500 $ — $ — $ 4,500 Money market mutual funds 143,122 — — 143,122 Commercial paper 25,913 — — 25,913 U.S. Treasury securities 15,991 45 — 16,036 Corporate debt securities 45,172 — (64 ) 45,108 Total $ 234,698 $ 45 $ (64 ) $ 234,679 As of December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Repurchase agreements $ 14,000 $ — $ — $ 14,000 Asset-backed securities 14,866 10 — 14,876 Money market mutual funds 39,524 — — 39,524 Commercial paper 41,259 — — 41,259 U.S. Treasury securities 59,926 45 — 59,971 Corporate debt securities 92,380 128 — 92,508 Total $ 261,955 $ 183 $ — $ 262,138 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 months ended March 31, 2020 and 2019. The contractual maturity dates of all of the Company’s investments are less than one year.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2020
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 March 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 $ 143,122 $ 143,122 $ — $ — Total cash equivalents $ 143,122 $ 143,122 $ — $ — Short-term investments: Asset-backed securities $ 4,500 $ — $ 4,500 $ — Commercial paper 25,913 — 25,913 — U.S. Treasury securities 16,036 — 16,036 — Corporate debt securities 45,108 — 45,108 — Total short-term investments $ 91,557 $ — $ 91,557 $ — Total financial assets $ 234,679 $ 143,122 $ 91,557 $ — Description December 31, 2019 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 $ 39,524 $ 39,524 $ — $ — Repurchase agreements 14,000 — 14,000 — Total cash equivalents $ 53,524 $ 39,524 $ 14,000 $ — Short-term investments: Asset-backed securities $ 14,876 $ — $ 14,876 $ — Commercial paper 41,259 — 41,259 — U.S. Treasury securities 59,971 — 59,971 — Corporate debt securities 92,508 — 92,508 — Total short-term investments $ 208,614 $ — $ 208,614 $ — Total financial assets $ 262,138 $ 39,524 $ 222,614 $ — 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 months ended March 31, 2020 and 2019.

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
	Mar. 31, 2020
Property Plant And Equipment [Abstract]
PROPERTY AND EQUIPMENT, NET	5. PROPERTY AND EQUIPMENT, NET Property and equipment, net consists of the following: As of March 31, 2020 December 31, 2019 (in thousands) Laboratory equipment $ 11,697 $ 10,837 Manufacturing equipment 7,479 4,911 Computers and purchased software 1,215 640 Furniture and fixtures 1,363 1,363 Leasehold improvements 29,961 30,862 Assets not yet in service 1,168 2,513 Property and equipment, at cost 52,883 51,126 Less: accumulated depreciation and amortization (10,317 ) (8,410 ) Property and equipment, net $ 42,566 $ 42,716 Depreciation expense for the three months ended March 31, 2020 was approximately $1.9 million, compared to $1.3 million for the three months ended March 31, 2019. There were no disposals of property and equipment during the three months ended March 31, 2020 and 2019. Leasehold improvements consist primarily of costs associated with the buildout of the Company’s research and development, manufacturing and general office space in Bedford, Massachusetts.

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	3 Months Ended
	Mar. 31, 2020
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 March 31, 2020 December 31, 2019 (in thousands) Accrued research and development expenses $ 3,808 $ 2,258 Accrued compensation and benefits 2,254 4,551 Accrued professional fees 642 542 Accrued unvested common stock subject to repurchase 25 37 Accrued other 220 256 Total accrued expenses and other liabilities $ 6,949 $ 7,644

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2020
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 term. On August 13, 2018, the Company entered into a sublease agreement for the entire leased premises. The rent commencement date of the sublease was December 2018, and the sublease will terminate on the scheduled termination date of the original lease. Under the terms of the sublease, the subtenant is obligated to pay the Company aggregate base rent of approximately $2.7 million over the term of the sublease in addition to passthrough of operating expenses and real estate taxes charged by the landlord. 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 term. Rent became due under the lease in two phases; rent on the first 46,000 square feet started in September 2018 and rent on the remaining 21,000 square feet started in March 2019. The initial annual base rent is $39.50 per square foot and will increase by three percent annually. The Company is obligated to pay, on a pro-rata basis, real estate taxes and operating costs related to the premises. The lease agreement entered into in December 2017 allowed for a tenant improvement allowance not to exceed $10.9 million, which the Company received in full, to be applied to the total cost of tenant improvements to the leased premises. The unamortized balance of the tenant improvement allowance was included in deferred rent and has been recorded as a reduction to the operating right-of-use asset upon the adoption of the new leasing standards. The Company maintains letters of credit, secured by restricted cash, for security deposits totaling $1.3 million as of March 31, 2020 and December 31, 2019, respectively, in conjunction with its current leases. The following table summarizes operating lease costs as well as sublease income for the three months ended March 31, 2020: Three months ended March 31, 2020 Amount (in thousands) Operating lease costs $ 623 Sublease income (228 ) Net lease cost $ 395 Rent expense, net of amortization of the deferred rent incentives was $0.2 million for the three months ended March 31, 2019. The maturities of our operating lease liabilities and minimum lease payments as of March 31, 2020 were as follows: For the Years Ending December 31, Amount (in thousands) 2020 $ 2,863 2021 3,834 2022 2,987 2023 3,077 2024 3,169 Thereafter 7,197 Total undiscounted lease payments $ 23,127 Less: imputed interest (5,975 ) Present value of operating lease liabilities $ 17,152 The following table summarizes the lease term and discount rate as of March 31, 2020: As of March 31, 2020 Weighted-average remaining lease term (years) Operating leases 6.4 Weighted-average discount rate Operating leases 9.9 % The following table summarizes cash paid for amounts included in the measurement of the Company’s operating lease liabilities for the three months ended March 31, 2020. Three months ended March 31, 2020 Amount (in thousands) Cash paid for amounts included in the measurement of lease liabilities $ 943 Operating cash flows from operating leases $ 943

Stock Incentive Plans

Stock Incentive Plans	3 Months Ended
	Mar. 31, 2020
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 period and expire ten years from the date of grant. Certain options provide for accelerated vesting if there is a change in control, as defined in the 2015 Plan. At March 31, 2020, there were no additional shares available for future grant under the 2015 Plan. 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 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, 2020, an additional 1,804,670 shares were added to the 2018 Plan, representing 4% of total common shares outstanding at December 31, 2019. At March 31, 2020, there were 2,802,656 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, 2020, an additional 451,167 shares were added to the 2018 ESPP, representing 1% of total common shares outstanding at December 31, 2019. At March 31, 2020, there were 1,084,028 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 three months ended March 31, 2020, 39,471 shares were issued under the 2018 ESPP for aggregate proceeds to the Company of $0.5 million. During the three months ended March 31, 2019, 28,311 shares were issued under the 2018 ESPP for aggregate proceeds to the Company of $0.4 million. The Company did not record any stock-based compensation pursuant to the 2018 ESPP during the three months ended March 31, 2020 and 2019, as the amounts estimated were insignificant in each such period. 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 fair value of each option award is estimated 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 March 31, 2020 2019 Expected volatility 63.2% — 63.5% 64.0% — 64.7% Weighted-average risk-free interest rate 0.74% — 1.73% 2.48% — 2.60% Expected dividend yield — % — % Expected term (in years) 6.25 6.25 Underlying common stock fair value $17.00 — $21.75 $21.43 — $28.13 The Company recorded stock-based compensation expense related to stock options and shares purchased under the 2018 ESPP as follows: Three months ended March 31, 2020 2019 (in thousands) Research and development $ 1,313 $ 558 General and administrative 1,570 685 $ 2,883 $ 1,243 As of March 31, 2020, there was $36.6 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 2.9 years at March 31, 2020. A summary of option activity under the Plans during the three months ended March 31, 2020 is as follows: 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, 2020 4,843,018 $ 14.78 8.5 $ 33,809 Granted 666,550 $ 21.40 Exercised (34,311 ) $ 3.93 Cancelled/Forfeited (27,442 ) $ 19.57 Outstanding at March 31, 2020 5,447,815 $ 15.63 8.5 $ 19,541 Vested and expected to vest at March 31, 2020 5,447,815 $ 15.63 8.5 $ 19,541 Exercisable at March 31, 2020 1,925,388 $ 10.05 7.5 $ 14,249 The total intrinsic value of options exercised during the three months ended March 31, 2020 and 2019 was $0.5 million and $0.4 million, respectively. The weighted-average grant date fair value per share of options granted during the three months ended March 31, 2020 and 2019 was $12.47 and $15.80, 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, 2020 21,666 Vested (15,542 ) Issued — Repurchased — Unvested shares - March 31, 2020 6,124 As of March 31, 2020 and December 31, 2019, 6,124 shares and 21,666 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 March 31, 2020 and December 31, 2019.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2020
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 and outstanding common stock options, 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 March 31, 2020 and 2019, 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 March 31, 2020 2019 Unvested common stock from early exercise of options 6,124 119,705 Stock options to purchase common stock 5,447,815 3,632,821 Total 5,453,939 3,752,526

Collaboration and License Agree

Collaboration and License Agreement	3 Months Ended
	Mar. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
COLLABORATION AND LICENSE AGREEMENT	10. COLLABORATION AND LICENSE AGREEMENT Summary of Agreement In November 2017, the Company entered into a collaboration and license agreement (the “Collaboration Agreement”) with Novartis Institutes of BioMedical Research, Inc. (“Novartis”) 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. The Company continues to work with Novartis to identify new targets for the partnership based on the existing exploratory research component of the agreement and the collaboration on ophthalmic programs also continues. Under the terms of the Collaboration Agreement, taking into account Novartis’ election to discontinue the sickle cell disease program, 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 the ophthalmic targets. Upon entering into the Collaboration Agreement, the Company received an upfront, nonrefundable payment of $35.0 million and issued additional shares of its Series B preferred stock to Novartis for consideration of $10.0 million. The Company recorded the Series B preferred stock at its estimated fair value of $11.7 million, including $1.7 million of the upfront payment, and allocated the remaining $33.3 million of the upfront payment to the Collaboration Agreement. The Collaboration Agreement consists of a research term, where the Company and Novartis are collaborating to perform research and conduct preclinical development to identify candidates that modulate the ophthalmic targets. The Collaboration Agreement also includes exploratory work on the applicability of the gene-editing technology with respect to other gene targets. The Company’s obligation to perform such exploratory research concludes in November 2020. Novartis may select up to two ophthalmic targets, with limited substitution rights. The Company is responsible for the manufacturing of proprietary research grade human hematopoietic stem cell derived adeno-associated virus vectors (“AAVHSCs”) during the research term. Research activities performed by the Company are being reimbursed at a full-time equivalent rate (“FTE”) and manufacturing activities will be reimbursed at cost, up to a maximum of $17.0 million during the research term, as specified and defined in the Collaboration Agreement. Novartis is required to pay the Company a target fee of $5.0 million for each target that meets certain success criteria during the research term (the “target fee trigger date”), up to a maximum of two targets. The research term will continue for five years from the effective date of the Collaboration Agreement. Pursuant to the Collaboration Agreement, the Company will also participate on a joint steering committee and a joint manufacturing subcommittee, with equal representation from both the Company and Novartis. Novartis has the exclusive right to develop and commercialize up to two candidates or products arising from the research activities. Subject to certain limitations pursuant to the terms of the Collaboration Agreement, Novartis will fund all development and commercialization costs. The Company will be responsible for manufacturing candidates and products for Novartis during the development and commercialization terms. The Company’s manufacturing activities will be reimbursed at cost during the development term and at cost plus a margin during the commercialization term, as defined in the Collaboration Agreement. If the Company is not able to manufacture candidates or products that meet the quality or quantity requirements of Novartis, then Novartis shall have the right to designate a third-party contract manufacturer or manufacture such candidates or products itself. In accordance with the Collaboration Agreement, taking into consideration Novartis’ election to discontinue the sickle cell disease program, the Company is also eligible to receive up to a total of $530.0 million in milestone payments, including up to $180.0 million in development milestone payments, up to $170.0 million in regulatory milestone payments and up to $180.0 million in commercial milestone payments, with respect to the licensed products. The Company is also eligible to earn tiered royalties on net sales of licensed products by Novartis, its affiliates or sublicensees, ranging from mid single-digit, to sub-teen double-digit percentages, and such royalties are potentially subject to various reductions and offsets. If any of the exploratory research efforts are advanced into formal research and development programs, the parties will negotiate the economics including potential milestone payments for such programs at that time. Unless earlier terminated, the Collaboration Agreement will continue on a target-by-target basis until the expiration of all applicable royalty terms with respect to all products that modulate such target on a country-by-country-basis. Either party may terminate the agreement on a target-by-target basis for the other party’s material breach with respect to such target, or in the event of the other party’s bankruptcy. Novartis may terminate the agreement for convenience on a target-by-target basis. The Company may terminate the agreement if Novartis files, or joins a third party in filing or maintaining, a patent challenge against certain of the patent rights licensed to Novartis under the terms of the agreement. There are no performance, cancellation, termination or refund provisions in the arrangement that contain material financial consequences to the Company. Revenue Recognition The Company 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. Since the option for Novartis to obtain manufacturing during the commercialization term from the Company is at a price that would reflect the standalone selling price, the option does not provide Novartis with a material right and is therefore not a performance obligation under the contract. The Company has concluded that the research, development and commercialization and manufacturing licenses are not distinct from the research activities and manufacturing during the research and development term as Novartis cannot benefit from the licenses without the Company performing the research and manufacturing services. These services are so specialized and rely on the Company’s expertise in gene editing technologies not available in the marketplace. As a result, these licenses have been combined with the research activities, service on the joint committees and the manufacturing during the research and development terms as a single performance obligation. The transaction price consists of the $33.3 million non-refundable upfront payment, net of amounts classified as equity, and projected reimbursable research and manufacturing activities, which have been estimated using the expected value method. None of the target fees or development and regulatory milestone payments have been included in the transaction price, as all are fully constrained. As part of the evaluation of the constraint, the Company considered numerous factors, including the fact that achievement of the milestones is outside of the Company’s control and is contingent upon the success of the Company’s preclinical studies, clinical trials, Novartis’ efforts and the receipt of regulatory approval. In addition, the Company has determined that the commercial milestones and sales-based royalties will be recognized when the related sales occur as they were determined to relate predominately to the licenses transferred to Novartis, and therefore have also been excluded from the transaction price. The Company will re-evaluate the transaction price, including estimated variable consideration included in the transaction price and all constrained amounts, in each reporting period and as uncertain events are resolved or other changes in circumstances occur. The Company identified only one performance obligation in the Collaboration Agreement. Therefore, the Company will allocate the transaction price at the onset of the contract to the single performance obligation. The Company recognizes revenue 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 March 31, 2020, the aggregate amount of the transaction price related to the unsatisfied portion of the performance obligation is $30.7 million, and the Company will recognize this revenue as the research and manufacturing activities are performed, which is expected to occur over a period of time that is estimated will conclude in 2027. In estimating the total costs to satisfy its single performance obligation pursuant to the Novartis agreement, the Company is required to make significant estimates including the expected time it will take to fulfill the performance obligation, which currently is expected to be approximately ten years from the date the Collaboration Agreement was entered into, and the expected costs associated with manufacturing during the research and development terms for a novel manufacturing process as well as the probability of success of a target to move into the development phase. The Company has made estimates of such costs utilizing its experience with similar product candidates, however not identical to those in the Collaboration Agreement. In making such estimates, significant judgment is required to evaluate those key assumptions related to cost estimates. The cumulative effect of revisions to the total estimated costs to complete the Company’s single performance obligation will be recorded in the period in which the changes are identified and amounts can be reasonably estimated. While such revisions will have no impact on the Company’s reported cash flows, a significant change in these assumptions and estimates could have a material impact on the timing and amount of revenue recognized in future periods. During the three months ended March 31, 2020, the Company recognized revenue under the Collaboration Agreement of $0.6 million, of which $0.2 million was included in deferred revenue at the beginning of the period. During the three months ended March 31, 2019, the Company recognized revenue under the Collaboration Agreement of $0.3 million, of which $0.1 million was included in deferred revenue at the beginning of the period. As of March 31, 2020 and December 31, 2019, there was approximately $30.7 million and $31.0 million of deferred revenue related to the Collaboration Agreement, respectively. In addition, as of March 31, 2020 and December 31, 2019, the Company has recorded accounts receivable of $0.4 million 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. * * * * * *

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2020
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, 2019, included in the Company's Annual Report on Form 10-K on file with the SEC. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position as of March 31, 2020, and consolidated results of operations for the three months ended March 31, 2020 and 2019, and cash flows for the three months ended March 31, 2020 and 2019. Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2020.
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: March 31, 2020 2019 (in thousands) Cash and cash equivalents $ 143,372 $ 40,839 Restricted cash 1,274 1,274 Total cash, cash equivalents and restricted cash $ 144,646 $ 42,113
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 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. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion, together with interest on securities, are included in interest income on 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.
Deferred Offering Costs	Deferred 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 — In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which eliminated the tests for lease classification under prior U.S. GAAP and requires lessees to recognize right-of-use assets and related lease liabilities on the balance sheet. The FASB subsequently issued the following amendments to ASU 2016-02 that have the same effective date and transition date: ASU No. 2018-01 - Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842 , ASU No. 2018-10 - Codification Improvements to Topic 842 , Leases , ASU No. 2018-11 – Leases (Topic 842): Targeted Improvements , ASU No. 2018-20 – Leases (Topic 842): Narrow-Scope Improvements for Lessors , and ASU No. 2019-01 – Leases (Topic 842): Codification Improvements . The Company adopted these amendments with ASU 2016-02 (collectively, the new leasing standards) effective January 1, 2020. The Company adopted the new leasing standards using the modified retrospective approach, as of January 1, 2020, with no restatement of prior periods or cumulative adjustments to retained earnings. The Company elected the package of practical expedients permitted under the transition guidance within the new standard, which, among other things, allows the Company to carry forward the historical lease classification. In addition, the Company elected the practical expedient not to apply the recognition requirements in the leasing standards to short-term leases (a lease that at commencement date has a lease term of 12 months or less and does not contain a purchase option that it is reasonably certain to exercise) and the practical expedient that permits lessees to make an accounting policy election (by class of underlying asset) to not separate lease components of a contract from non-lease components. 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. 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. Right-of-use assets are periodically evaluated for impairment. The expected lease term for those leases commencing prior to January 1, 2020 did not change with the adoption of the new leasing standards. The expected lease term for leases commencing after the adoption of the new leasing standards 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. As a result of the adoption of the new leasing standards, on January 1, 2020, the Company recorded a right-of-use asset of $6.8 million, operating lease liabilities of $17.7 million and the derecognition of deferred rent of $10.9 million originally accounted for under legacy guidance. The adoption did not have a material impact on the condensed consolidated statement of operations. For additional information on the adoption of the new leasing standards, refer to Note 7. January 1, 2020 (in thousands) Prior to adoption of new leasing standards Adjustment for adoption of new leasing standards As Adjusted Right-of-use assets (1)(2) $ — $ 6,835 $ 6,835 Deferred rent (2) $ 1,313 $ (1,313 ) $ — Deferred rent, net of current portion (2) $ 9,544 $ (9,544 ) $ — Operating lease liabilities (3) $ — $ 2,251 $ 2,251 Operating lease liabilities, net of current portion (3) $ — $ 15,441 $ 15,441 (1) Represents capitalization of operating right-of-use assets (2) Represents reclassification of deferred rent and incentives as a reduction to operating right-of-use assets (3) Represents recognition of operating lease liabilities
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 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 February 2016, the FASB issued new leasing standards to increase transparency and comparability among organization related to their leasing activities. For additional information on the adoption of the new leasing standards, please refer to section titled “Leases” above, and Note 7. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ,

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2020
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: March 31, 2020 2019 (in thousands) Cash and cash equivalents $ 143,372 $ 40,839 Restricted cash 1,274 1,274 Total cash, cash equivalents and restricted cash $ 144,646 $ 42,113
Impact of Adoption of Previously Reported Amounts of Financial Statement	January 1, 2020 (in thousands) Prior to adoption of new leasing standards Adjustment for adoption of new leasing standards As Adjusted Right-of-use assets (1)(2) $ — $ 6,835 $ 6,835 Deferred rent (2) $ 1,313 $ (1,313 ) $ — Deferred rent, net of current portion (2) $ 9,544 $ (9,544 ) $ — Operating lease liabilities (3) $ — $ 2,251 $ 2,251 Operating lease liabilities, net of current portion (3) $ — $ 15,441 $ 15,441 (1) Represents capitalization of operating right-of-use assets (2) Represents reclassification of deferred rent and incentives as a reduction to operating right-of-use assets (3) Represents recognition of operating lease liabilities

Short-Term Investments (Tables)

Short-Term Investments (Tables)	3 Months Ended
	Mar. 31, 2020
Investments Disclosure [Abstract]
Summary of Investments Included in Cash Equivalents and Short Term Investments	The following table summarizes the Company’s investments, which are included in cash equivalents and short-term investments: As of March 31, 2020 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Asset-backed securities $ 4,500 $ — $ — $ 4,500 Money market mutual funds 143,122 — — 143,122 Commercial paper 25,913 — — 25,913 U.S. Treasury securities 15,991 45 — 16,036 Corporate debt securities 45,172 — (64 ) 45,108 Total $ 234,698 $ 45 $ (64 ) $ 234,679 As of December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Repurchase agreements $ 14,000 $ — $ — $ 14,000 Asset-backed securities 14,866 10 — 14,876 Money market mutual funds 39,524 — — 39,524 Commercial paper 41,259 — — 41,259 U.S. Treasury securities 59,926 45 — 59,971 Corporate debt securities 92,380 128 — 92,508 Total $ 261,955 $ 183 $ — $ 262,138

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2020
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 March 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 $ 143,122 $ 143,122 $ — $ — Total cash equivalents $ 143,122 $ 143,122 $ — $ — Short-term investments: Asset-backed securities $ 4,500 $ — $ 4,500 $ — Commercial paper 25,913 — 25,913 — U.S. Treasury securities 16,036 — 16,036 — Corporate debt securities 45,108 — 45,108 — Total short-term investments $ 91,557 $ — $ 91,557 $ — Total financial assets $ 234,679 $ 143,122 $ 91,557 $ — Description December 31, 2019 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 $ 39,524 $ 39,524 $ — $ — Repurchase agreements 14,000 — 14,000 — Total cash equivalents $ 53,524 $ 39,524 $ 14,000 $ — Short-term investments: Asset-backed securities $ 14,876 $ — $ 14,876 $ — Commercial paper 41,259 — 41,259 — U.S. Treasury securities 59,971 — 59,971 — Corporate debt securities 92,508 — 92,508 — Total short-term investments $ 208,614 $ — $ 208,614 $ — Total financial assets $ 262,138 $ 39,524 $ 222,614 $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
	Mar. 31, 2020
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consists of the following: As of March 31, 2020 December 31, 2019 (in thousands) Laboratory equipment $ 11,697 $ 10,837 Manufacturing equipment 7,479 4,911 Computers and purchased software 1,215 640 Furniture and fixtures 1,363 1,363 Leasehold improvements 29,961 30,862 Assets not yet in service 1,168 2,513 Property and equipment, at cost 52,883 51,126 Less: accumulated depreciation and amortization (10,317 ) (8,410 ) Property and equipment, net $ 42,566 $ 42,716

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	3 Months Ended
	Mar. 31, 2020
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consist of the following: As of March 31, 2020 December 31, 2019 (in thousands) Accrued research and development expenses $ 3,808 $ 2,258 Accrued compensation and benefits 2,254 4,551 Accrued professional fees 642 542 Accrued unvested common stock subject to repurchase 25 37 Accrued other 220 256 Total accrued expenses and other liabilities $ 6,949 $ 7,644

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Schedule of Operating Lease Cost and Sub Lease Income	The following table summarizes operating lease costs as well as sublease income for the three months ended March 31, 2020: Three months ended March 31, 2020 Amount (in thousands) Operating lease costs $ 623 Sublease income (228 ) Net lease cost $ 395
Schedule of Operating Lease Liabilities and Minimum Lease Payments	The maturities of our operating lease liabilities and minimum lease payments as of March 31, 2020 were as follows: For the Years Ending December 31, Amount (in thousands) 2020 $ 2,863 2021 3,834 2022 2,987 2023 3,077 2024 3,169 Thereafter 7,197 Total undiscounted lease payments $ 23,127 Less: imputed interest (5,975 ) Present value of operating lease liabilities $ 17,152
Schedule of Lease Term and Discount Rate	The following table summarizes the lease term and discount rate as of March 31, 2020: As of March 31, 2020 Weighted-average remaining lease term (years) Operating leases 6.4 Weighted-average discount rate Operating leases 9.9 %
Schedule of Operating Lease Liabilities	The following table summarizes cash paid for amounts included in the measurement of the Company’s operating lease liabilities for the three months ended March 31, 2020. Three months ended March 31, 2020 Amount (in thousands) Cash paid for amounts included in the measurement of lease liabilities $ 943 Operating cash flows from operating leases $ 943

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	3 Months Ended
	Mar. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model	Error extracting Word content The assumptions used in the Black-Scholes option pricing model are as follows: Three months ended March 31, 2020 2019 Expected volatility 63.2% — 63.5% 64.0% — 64.7% Weighted-average risk-free interest rate 0.74% — 1.73% 2.48% — 2.60% Expected dividend yield — % — % Expected term (in years) 6.25 6.25 Underlying common stock fair value $17.00 — $21.75 $21.43 — $28.13
Schedule of Stock-Based Compensation Expense	The Company recorded stock-based compensation expense related to stock options and shares purchased under the 2018 ESPP as follows: Three months ended March 31, 2020 2019 (in thousands) Research and development $ 1,313 $ 558 General and administrative 1,570 685 $ 2,883 $ 1,243
Summary of Option Activity under Plans	A summary of option activity under the Plans during the three months ended March 31, 2020 is as follows: 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, 2020 4,843,018 $ 14.78 8.5 $ 33,809 Granted 666,550 $ 21.40 Exercised (34,311 ) $ 3.93 Cancelled/Forfeited (27,442 ) $ 19.57 Outstanding at March 31, 2020 5,447,815 $ 15.63 8.5 $ 19,541 Vested and expected to vest at March 31, 2020 5,447,815 $ 15.63 8.5 $ 19,541 Exercisable at March 31, 2020 1,925,388 $ 10.05 7.5 $ 14,249
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, 2020 21,666 Vested (15,542 ) Issued — Repurchased — Unvested shares - March 31, 2020 6,124

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2020
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 March 31, 2020 and 2019, 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 March 31, 2020 2019 Unvested common stock from early exercise of options 6,124 119,705 Stock options to purchase common stock 5,447,815 3,632,821 Total 5,453,939 3,752,526

Nature of Business and Basis _2

Nature of Business and Basis of Presentation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Mar. 12, 2020	Apr. 12, 2019	Apr. 01, 2019	Apr. 02, 2018	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Apr. 30, 2019
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
Common Stock, capacity terminated								$ 17,600
Net loss					$ 35,331	$ 23,852
Accumulated deficit					235,005		$ 199,674
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold				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
ATM
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings					$ 150,000
ATM | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold					0
ATM | Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings	$ 150,000		$ 100,000
Follow-On Offering | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold		5,555,556
Shares issued price per share		$ 22.50
Net proceeds after deducting underwriting discounts and commissions and offering expenses		$ 116,900
Underwriters Option | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold		833,333
Shares issued price per share		$ 22.50
Net proceeds after deducting underwriting discounts and commissions and offering expenses		$ 17,600

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Accounting Policies [Abstract]
Cash and cash equivalents	$ 143,372	$ 53,774	$ 40,839
Restricted cash	1,274	$ 1,274	1,274
Total cash, cash equivalents and restricted cash	$ 144,646		$ 42,113

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020
Summary Of Significant Accounting Policies [Line Items]
Right-of-use assets	$ 6,615	$ 6,835	[1],[2]
Operating lease liabilities	$ 17,152	2,251	[3]
ASU 2016-02
Summary Of Significant Accounting Policies [Line Items]
Right-of-use assets		6,800
Operating lease liabilities		17,700
Deferred rent		$ 10,900
[1]	Represents capitalization of operating right-of-use assets
[2]	Represents reclassification of deferred rent and incentives as a reduction to operating right-of-use assets
[3]	Represents recognition of operating lease liabilities

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Impact of Adoption of Previously Reported Amounts of Condensed Consolidated Balance Sheets (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Jan. 01, 2020		Dec. 31, 2019
Right-of-use assets		$ 6,615	$ 6,835	[1],[2]
Deferred rent					$ 1,313
Operating lease liabilities		17,152	2,251	[3]
Operating lease liabilities		$ 2,331	15,441	[3]
Prior to Adoption of New Leasing Standards
Deferred rent	[2]		1,313
Deferred rent	[2]		9,544
Adjustment for Adoption of New Leasing Standards
Right-of-use assets	[1],[2]		6,835
Deferred rent	[2]		1,313
Deferred rent	[2]		(9,544)
Operating lease liabilities	[3]		2,251
Operating lease liabilities	[3]		$ 15,441
[1]	Represents capitalization of operating right-of-use assets
[2]	Represents reclassification of deferred rent and incentives as a reduction to operating right-of-use assets
[3]	Represents recognition of operating lease liabilities

Short-Term Investments - Summar

Short-Term Investments - Summary of Investments Included in Cash Equivalents and Short Term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	$ 234,698	$ 261,955
Cash equivalents and short-term investments, Unrealized Gains	45	183
Cash equivalents and short-term investments, Unrealized Losses	(64)
Cash equivalents and short-term investments, Fair Value	234,679	262,138
Money Market Mutual Funds
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	143,122	39,524
Cash equivalents and short-term investments, Fair Value	143,122	39,524
Repurchase Agreements
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost		14,000
Cash equivalents and short-term investments, Fair Value		14,000
Asset-backed Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	4,500	14,866
Cash equivalents and short-term investments, Unrealized Gains		10
Cash equivalents and short-term investments, Fair Value	4,500	14,876
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	25,913	41,259
Cash equivalents and short-term investments, Fair Value	25,913	41,259
Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	45,172	92,380
Cash equivalents and short-term investments, Unrealized Gains		128
Cash equivalents and short-term investments, Unrealized Losses	(64)
Cash equivalents and short-term investments, Fair Value	45,108	92,508
U.S Treasury Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	15,991	59,926
Cash equivalents and short-term investments, Unrealized Gains	45	45
Cash equivalents and short-term investments, Fair Value	$ 16,036	$ 59,971

Short-Term Investments - Additi

Short-Term Investments - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash Cash Equivalents And Available For Sale Securities [Abstract]
Realized gains and losses on available-for-sale securities	$ 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 ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	$ 91,557	$ 208,614
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	143,122	53,524
Short-term investments, fair value	91,557	208,614
Financial assets, fair value	234,679	262,138
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	143,122	39,524
Fair Value, Measurements, Recurring | Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value		14,000
Fair Value, Measurements, Recurring | Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	4,500	14,876
Fair Value, Measurements, Recurring | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	25,913	41,259
Fair Value, Measurements, Recurring | U.S Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	16,036	59,971
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	45,108	92,508
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	143,122	39,524
Financial assets, fair value	143,122	39,524
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	143,122	39,524
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value		14,000
Short-term investments, fair value	91,557	208,614
Financial assets, fair value	91,557	222,614
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value		14,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	4,500	14,876
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	25,913	41,259
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	16,036	59,971
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	$ 45,108	$ 92,508

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Transfers between fair value measure levels	$ 0	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment, at cost	$ 52,883	$ 51,126
Less: accumulated depreciation and amortization	(10,317)	(8,410)
Property and equipment, net	42,566	42,716
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, at cost	11,697	10,837
Manufacturing Equipment
Property Plant And Equipment [Line Items]
Property and equipment, at cost	7,479	4,911
Computers and Purchased Software
Property Plant And Equipment [Line Items]
Property and equipment, at cost	1,215	640
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, at cost	1,363	1,363
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, at cost	29,961	30,862
Assets Not Yet in Service
Property Plant And Equipment [Line Items]
Property and equipment, at cost	$ 1,168	$ 2,513

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Property Plant And Equipment [Abstract]
Depreciation	$ 1,907,000	$ 1,266,000
Disposal of property and equipment	$ 0	$ 0

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued research and development expenses	$ 3,808	$ 2,258
Accrued compensation and benefits	2,254	4,551
Accrued professional fees	642	542
Accrued unvested common stock subject to repurchase	25	37
Accrued other	220	256
Total accrued expenses and other liabilities	$ 6,949	$ 7,644

Commitment and Contingencies -

Commitment and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended
	Dec. 31, 2017USD ($)ft²Phase	Sep. 30, 2016	Mar. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Commitment And Contingencies [Line Items]
Rent expense, net			$ 200,000
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	$ 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 Cost and Sub Lease Income (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020USD ($)
Commitments And Contingencies Disclosure [Abstract]
Operating lease costs	$ 623
Sublease income	(228)
Net lease cost	$ 395

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Operating Lease Liabilities and Minimum Lease Payments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	[1]
Commitments And Contingencies Disclosure [Abstract]
2020	$ 2,863
2021	3,834
2022	2,987
2023	3,077
2024	3,169
Thereafter	7,197
Total undiscounted lease payments	23,127
Less: imputed interest	(5,975)
Operating lease liabilities	$ 17,152	$ 2,251
[1]	Represents recognition of operating lease liabilities

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Lease Term and Discount Rate (Details)	Mar. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Weighted-average remaining lease term (years), Operating leases	6 years 4 months 24 days
Weighted-average discount rate, Operating leases	9.90%

Commitments and Contingencies_3

Commitments and Contingencies - Summary of Cash Paid for Company's Operating Lease Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020USD ($)
Commitments And Contingencies Disclosure [Abstract]
Cash paid for amounts included in the measurement of lease liabilities	$ 943
Operating cash flows from operating leases	$ 943

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2019	Mar. 31, 2018	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares issued to the plan				28,311
Aggregate proceeds from shares issued under the plan			$ 537	$ 396
Stock-based compensation			2,883	1,243
Total intrinsic value of options exercised			$ 500	$ 400
Weighted-average grant date fair value per share for options granted			$ 12.47	$ 15.80
Remaining shares subject to repurchase			6,124		21,666
Liability for unvested common shares subject to repurchase included in accrued expenses and other liabilities			$ 25		$ 37
Accrued Expenses and Other Liabilities
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Liability for unvested common shares subject to repurchase included in accrued expenses and other liabilities			$ 100		$ 100
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
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	1,804,670
Number of shares outstanding available for future grant			2,802,656
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	451,167
Number of shares outstanding available for future grant			1,084,028
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			39,471
Aggregate proceeds from shares issued under the plan			$ 500
Stock-based compensation			0	$ 0
2015 and 2018 Stock Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense			$ 36,600
Unrecognized compensation expense estimated to be recognized over period			2 years 10 months 24 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
	Mar. 31, 2020	Mar. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	63.20%	64.00%
Expected volatility, maximum	63.50%	64.70%
Weighted-average risk-free interest rate, minimum	0.74%	2.48%
Weighted-average risk-free interest rate, maximum	1.73%	2.60%
Expected term (in years)	6 years 3 months	6 years 3 months
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Underlying common stock fair value	$ 17	$ 21.43
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Underlying common stock fair value	$ 21.75	$ 28.13

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 2,883	$ 1,243
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	1,313	558
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 1,570	$ 685

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Option Activity under Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding at Beginning Balance	4,843,018
Number of Options, Granted	666,550
Number of Options, Exercised	(34,311)
Number of Options, Cancelled/Forfeited	(27,442)
Number of Options, Outstanding at Ending Balance	5,447,815	4,843,018
Number of Options, Vested and Expected to vest at March 31, 2020	5,447,815
Number of Options, Exercisable at March 31, 2020	1,925,388
Weighted-Average Exercise Price per Share, Outstanding at Beginning Balance	$ 14.78
Weighted-Average Exercise Price per Share, Granted	21.40
Weighted-Average Exercise Price per Share, Exercised	3.93
Weighted-Average Exercise Price per Share, Cancelled/Forfeited	19.57
Weighted-Average Exercise Price per Share, Outstanding at Ending Balance	15.63	$ 14.78
Weighted-Average Exercise Price per Share, Vested and Expected to vest at March 31, 2020	15.63
Weighted-Average Exercise Price Per Share, Exercisable at March 31, 2020	$ 10.05
Weighted Average Remaining Contractual Term, Outstanding	8 years 6 months	8 years 6 months
Weighted Average Remaining Contractual Term, Vested and Expected to vest at March 31, 2020	8 years 6 months
Weighted Average Remaining Contractual Term, Exercisable at March 31, 2020	7 years 6 months
Aggregate Intrinsic Value, Outstanding Ending Balance	$ 19,541	$ 33,809
Aggregate Intrinsic Value, Vested and Expected to vest at March 31, 2020	19,541
Aggregate Intrinsic Value, Exercisable at March 31, 2020	$ 14,249

Stock Incentive Plans - Summa_2

Stock Incentive Plans - Summary of Unvested Common Stock from Early Exercises Subject to Repurchase (Details)	3 Months Ended
	Mar. 31, 2020shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unvested shares, Beginning Balance	21,666
Vested	(15,542)
Unvested shares, Ending Balance	6,124

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	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	5,453,939	3,752,526
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	6,124	119,705
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	5,447,815	3,632,821

Collaboration and License Agr_2

Collaboration and License Agreement - Additional Information (Details)	1 Months Ended	3 Months Ended
	Nov. 30, 2017USD ($)TargetCandidateorProduct	Mar. 31, 2020USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)	Jul. 01, 2019USD ($)	Jan. 01, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement revenue recognized		$ 588,000	$ 270,000
Novartis
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront, nonrefundable payment received	$ 35,000,000
Allocated to collaboration agreement	33,300,000
Collaborative agreement target fee	$ 5,000,000
Collaboration agreement, research term	5 years
Novartis | Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement revenue recognized		600,000	$ 300,000
Deferred revenue		30,700,000		$ 31,000,000	$ 100,000	$ 200,000
Novartis | Collaborative Arrangement | Reimbursable Research and Development Costs
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable		400,000		$ 400,000
Novartis | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Transaction price		$ 33,300,000
Novartis | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets selected | Target	2
Collaboration agreement, reimbursement of manufacturing activities during research term	$ 17,000,000
Number of targets during research term | Target	2
Collaboration agreement number of candidates or products for develop and commercialize | CandidateorProduct	2
Collaboration agreement eligible milestone payments eligible to receive	$ 530,000,000
Collaboration agreement eligible development milestone payments eligible to receive	180,000,000
Collaboration agreement eligible regulatory milestone payments eligible to receive	170,000,000
Collaboration agreement eligible commercial milestone payments eligible to receive	180,000,000
Novartis | Series B Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront, nonrefundable payment received	1,700,000
Proceeds from issuance of shares of preferred stock	10,000,000
Preferred stock estimated fair value	$ 11,700,000

Collaboration and License Agr_3

Collaboration and License Agreement - Additional Information (Details1) - Novartis $ in Millions	Mar. 31, 2020USD ($)
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	$ 30.7
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	$ 30.7
Remaining performance obligation, expected timing of satisfaction, period	10 years