Q32 Bio (QTTB) 10-Q 8 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Aug. 05, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Period Focus	Q2
Document Fiscal Year Focus	2019
Entity Registrant Name	Homology Medicines, Inc.
Entity Central Index Key	0001661998
Trading Symbol	FIXX
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		43,966,197
Entity File Number	001-38433
Entity Tax Identification Number	473468154
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

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 115,566	$ 38,220
Short-term investments	180,230	176,517
Prepaid expenses and other current assets	2,973	6,948
Total current assets	298,769	221,685
Property and equipment, net	41,113	35,637
Restricted cash	1,274	1,772
Total assets	341,156	259,094
Current liabilities:
Accounts payable	8,334	15,732
Accrued expenses and other liabilities	5,306	5,040
Deferred rent	1,161	977
Deferred revenue	690	770
Total current liabilities	15,491	22,519
Non-current liabilities:
Deferred rent, net of current portion	10,326	9,470
Deferred revenue, net of current portion	30,597	30,750
Total liabilities	56,414	62,739
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of June 30, 2019 and December 31, 2018; no shares issued and outstanding at June 30, 2019 and December 31, 2018
Common stock, $0.0001 par value; 200,000,000 shares authorized as of June 30, 2019 and December 31, 2018; 43,958,295 and 37,509,815 shares issued as of June 30, 2019 and December 31, 2018, respectively; and 43,869,614 and 37,358,526 shares outstanding as of June 30, 2019 and December 31, 2018, respectively	4	3
Additional paid-in capital	430,432	292,187
Accumulated other comprehensive gain (loss)	186	(77)
Accumulated deficit	(145,880)	(95,758)
Total stockholders’ equity	284,742	196,355
Total liabilities and stockholders' equity	$ 341,156	$ 259,094

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
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	43,958,295	37,509,815
Common stock, shares outstanding	43,869,614	37,358,526

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue	$ 392	$ 1,395	$ 662	$ 2,524
Operating expenses:
Research and development	22,827	10,290	43,368	18,288
General and administrative	5,538	4,527	10,390	8,355
Total operating expenses	28,365	14,817	53,758	26,643
Loss from operations	(27,973)	(13,422)	(53,096)	(24,119)
Other income:
Interest income	1,703	996	2,974	1,476
Total other income	1,703	996	2,974	1,476
Net loss and net loss attributable to common stockholders-basic and diluted	$ (26,270)	$ (12,426)	$ (50,122)	$ (22,643)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.61)	$ (0.34)	$ (1.25)	$ (1.15)
Weighted-average common shares outstanding-basic and diluted	43,075,587	36,860,619	40,230,484	19,606,543

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (26,270)	$ (12,426)	$ (50,122)	$ (22,643)
Other comprehensive gain:
Change in unrealized gain (loss) on available for sale securities, net	180	38	263	25
Total other comprehensive gain	180	38	263	25
Comprehensive loss	$ (26,090)	$ (12,388)	$ (49,859)	$ (22,618)

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (UNAUDITED) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Balance balance at Dec. 31, 2017		$ 137,762
Beginning balance at Dec. 31, 2017	$ (39,454)			$ 800	$ (73)	$ (40,181)
Beginning balance, Shares at Dec. 31, 2017		127,199,705
Beginning balance, Shares at Dec. 31, 2017			2,637,011
Vesting of common stock from option exercises	16			16
Vesting of common stock from option exercises, Shares			34,137
Issuance of common stock from option exercises	69			69
Issuance of common stock from option exercises, Shares			26,204
Stock-based compensation	380			380
Other comprehensive (loss) gain	(13)				(13)
Net loss	(10,217)					(10,217)
Ending balance at Mar. 31, 2018	(49,219)			1,265	(86)	(50,398)
Ending balance, Shares at Mar. 31, 2018		127,199,705
Ending balance at Mar. 31, 2018		$ 137,762
Ending balance, Shares at Mar. 31, 2018			2,697,352
Balance balance at Dec. 31, 2017		$ 137,762
Beginning balance at Dec. 31, 2017	(39,454)			800	(73)	(40,181)
Beginning balance, Shares at Dec. 31, 2017		127,199,705
Beginning balance, Shares at Dec. 31, 2017			2,637,011
Other comprehensive (loss) gain	25
Net loss	(22,643)
Ending balance at Jun. 30, 2018	227,573		$ 4	290,441	(48)	(62,824)
Ending balance, Shares at Jun. 30, 2018			37,254,902
Balance balance at Dec. 31, 2017		$ 137,762
Beginning balance at Dec. 31, 2017	(39,454)			800	(73)	(40,181)
Beginning balance, Shares at Dec. 31, 2017		127,199,705
Beginning balance, Shares at Dec. 31, 2017			2,637,011
Net loss	(55,577)
Ending balance at Dec. 31, 2018	196,355		$ 3	292,187	(77)	(95,758)
Ending balance, Shares at Dec. 31, 2018			37,358,526
Balance balance at Mar. 31, 2018		$ 137,762
Beginning balance at Mar. 31, 2018	(49,219)			1,265	(86)	(50,398)
Beginning balance, Shares at Mar. 31, 2018		127,199,705
Beginning balance, Shares at Mar. 31, 2018			2,697,352
Conversion of convertible preferred stock into common stock upon initial public offering	137,762	$ (137,762)	$ 3	137,759
Conversion of convertible preferred stock into common stock upon initial public offering, Shares		127,199,705
Conversion of convertible preferred stock into common stock upon initial public offering, Shares			24,168,656
Issuance of common stock in initial public offering, net of discounts and issuance costs	150,843		$ 1	150,842
Issuance of common stock in initial public offering, net of discounts and issuance costs, Shares			10,350,000
Vesting of common stock from option exercises	(10)			(10)
Vesting of common stock from option exercises, Shares			27,364
Issuance of common stock from option exercises	16			16
Issuance of common stock from option exercises, Shares			11,530
Stock-based compensation	569			569
Other comprehensive (loss) gain	38				38
Net loss	(12,426)					(12,426)
Ending balance at Jun. 30, 2018	227,573		$ 4	290,441	(48)	(62,824)
Ending balance, Shares at Jun. 30, 2018			37,254,902
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
Stock-based compensation	1,243			1,243
Other comprehensive (loss) gain	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, 2018	$ 196,355		$ 3	292,187	(77)	(95,758)
Beginning balance, Shares at Dec. 31, 2018			37,358,526
Issuance of common stock from option exercises, Shares	31,280
Other comprehensive (loss) gain	$ 263
Net loss	(50,122)
Ending balance at Jun. 30, 2019	284,742		$ 4	430,432	186	(145,880)
Ending balance, Shares at Jun. 30, 2019			43,869,614
Beginning balance at Mar. 31, 2019	174,264		$ 3	293,865	6	(119,610)
Beginning balance, Shares at Mar. 31, 2019			37,437,297
Issuance of common stock in follow-on offering, net of discounts and issuance costs	134,525		$ 1	134,524
Issuance of common stock in follow-on offering, net of discounts and issuance costs, Shares			6,388,889
Vesting of common stock from option exercises	18			18
Vesting of common stock from option exercises, Shares			31,024
Issuance of common stock from option exercises	55			55
Issuance of common stock from option exercises, Shares			12,404
Stock-based compensation	1,970			1,970
Other comprehensive (loss) gain	180				180
Net loss	(26,270)					(26,270)
Ending balance at Jun. 30, 2019	$ 284,742		$ 4	$ 430,432	$ 186	$ (145,880)
Ending balance, Shares at Jun. 30, 2019			43,869,614

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (50,122)	$ (22,643)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	2,908	458
Stock-based compensation expense	3,213	949
Accretion of discount (amortization of premium) on short-term investments	(1,200)	111
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	3,975	(4,130)
Accounts payable	576	2,567
Accrued expenses and other liabilities	302	355
Deferred revenue	(233)	(938)
Deferred rent	1,040	2,480
Net cash used in operating activities	(39,541)	(20,791)
Cash flows from investing activities:
Purchases of short-term investments	(189,475)	(143,770)
Maturities of short-term investments	187,225	61,220
Purchases of property and equipment	(16,385)	(4,184)
Net cash used in investing activities	(18,635)	(86,734)
Cash flows from financing activities:
Proceeds from issuance of common stock in initial public offering, net of discounts and issuance costs		150,843
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs	134,552
Proceeds from issuance of common stock from option exercises	76	85
Proceeds from issuance of common stock pursuant to employee stock purchase plan	396
Net cash provided by financing activities	135,024	150,928
Net change in cash, cash equivalents and restricted cash	76,848	43,403
Cash, cash equivalents and restricted cash, beginning of period	39,992	53,347
Cash, cash equivalents and restricted cash, end of period	116,840	96,750
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	36	6
Property and equipment additions included in accounts payable	1,126	230
Deferred offering costs included in accounts payable and accrued expenses	$ 27
Series A Convertible Preferred Stock | Initial Public Offering
Supplemental disclosures of noncash investing and financing activities:
Conversion of convertible preferred stock into common stock upon initial public offering		42,994
Series B Convertible Preferred Stock | Initial Public Offering
Supplemental disclosures of noncash investing and financing activities:
Conversion of convertible preferred stock into common stock upon initial public offering		$ 94,768

Nature of Business and Basis of

Nature of Business and Basis of Presentation	6 Months Ended
	Jun. 30, 2019
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 clinical 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 1, 2019, the Company filed a Registration Statement on Form S-3 (File No. 333-230664) (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 in the aggregate amount of up to $350.0 million for a period up to three years from the date of the filing. The Shelf became effective on April 9, 2019. The Company also simultaneously entered into a sales agreement with Cowen and Company, LLC, as sales agent, 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 under the Shelf. As of June 30, 2019, no securities have been issued pursuant to the sales agreement. On April 12, 2019, pursuant to the Shelf, 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. To date, the Company has not generated any revenue from product sales and does not expect to generate any revenue from the sale of product in the foreseeable future. Through June 30, 2019, 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 six months ended June 30, 2019, the Company incurred a net loss of $50.1 million and as of June 30, 2019, has $145.9 million in accumulated deficit. The Company expects to incur additional operating losses and negative operating cash flows for the foreseeable future. Based on current projections, management believes that existing cash, cash equivalents and short-term investments will allow 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, 2018, included in the Company's Annual Report on Form 10-K on file with the SEC. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position as of June 30, 2019, and consolidated results of operations for the three and six months ended June 30, 2019 and 2018, and cash flows for the six months ended June 30, 2019 and 2018. Such adjustments are of a normal and recurring nature. The results of operations for the three and six months ended June 30, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
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, useful lives assigned to property and equipment, as well as the fair values of common stock and convertible preferred stock. The Company assesses estimates on an ongoing basis; however, actual results could materially differ from those estimates. Comprehensive Income (Loss) —Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. The Company’s only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments. Cash and Cash Equivalents and Restricted Cash —Cash and cash equivalents consist of standard checking accounts, money market accounts and certain investments. The Company considers all highly liquid investments with original or remaining maturities at the time of purchase of 90 days or less to be cash equivalents. Restricted cash consists of cash serving as collateral for letters of credit issued for security deposits for the Company’s facility leases in Bedford, Massachusetts. The following table provides a reconciliation of cash, cash equivalents and restricted cash to amounts shown in the condensed consolidated statements of cash flows: June 30, 2019 2018 (in thousands) Cash and cash equivalents $ 115,566 $ 94,960 Restricted cash 1,274 1,790 Total cash, cash equivalents and restricted cash $ 116,840 $ 96,750 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 (deficit) 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 (expense). 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— In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue (Topic 606): Revenue from Contracts with Customers (“ASU 2014-09”), which amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in FASB ASC Topic 605, Revenue Recognition (“ASC 605”), and creates a new 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, an entity 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 good 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 good 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. The Company adopted ASC 606 effective January 1, 2019, using the full retrospective transition method. Under this method, the Company revised its condensed consolidated financial statements for prior period amounts as if ASC 606 had been effective for such periods. The references "as revised" used herein refer to revisions of data for the three and six months ended June 30, 2019 and the year ended December 31, 2018 as a result of the adoption of ASC 606. The adoption of ASC 606 did not have an impact on periods prior to January 1, 2018. In addition to revisions to financial statement amounts and related tables, Note 10 has been revised to reflect the adoption of ASC 606. As part of the adoption, the Company reviewed its collaboration and license agreement with Novartis to determine the cumulative effect impact related to the adoption of ASC 606. (For a complete discussion of the accounting for the Company’s agreement with Novartis, see Note 10.) The adoption of ASC 606 resulted in a change to the pattern of revenue recognition whereby the Company expects to recognize revenue from its collaboration agreement with Novartis as costs are incurred, which likely will not occur evenly over the performance period as a result of applying the cost-to-cost method, in contrast to recognizing revenue on a straight-line basis over the estimated performance period under ASC 605. The impact on previously reported amounts as a result of the adoption of ASC 606 is as follows: Condensed Consolidated Balance Sheets December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Current portion of deferred revenue $ 770 $ 3,684 $ (2,914 ) Long-term portion of deferred revenue $ 30,750 $ 29,474 $ 1,276 Accumulated deficit $ (95,758 ) $ (97,396 ) $ 1,638 Condensed Consolidated Statements of Operations and Comprehensive Loss Three Months Ended June 30, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 1,395 $ 926 $ 469 Loss from operations $ (13,422 ) $ (13,891 ) $ 469 Net loss $ (12,426 ) $ (12,895 ) $ 469 Net loss per share attributable to common stockholders-basic and diluted $ (0.34 ) $ (0.35 ) $ 0.01 Six Months Ended June 30, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 2,524 $ 1,749 $ 775 Loss from operations $ (24,119 ) $ (24,894 ) $ 775 Net loss $ (22,643 ) $ (23,418 ) $ 775 Net loss per share attributable to common stockholders-basic and diluted $ (1.15 ) $ (1.19 ) $ 0.04 For the Year Ended December 31, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 5,322 $ 3,684 $ 1,638 Loss from operations $ (59,926 ) $ (61,564 ) $ 1,638 Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Net loss per share attributable to common stockholders-basic and diluted $ (1.95 ) $ (2.00 ) $ 0.05 Condensed Consolidated Statement of Cash Flows Six Months Ended June 30, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Net loss $ (22,643 ) $ (23,418 ) $ 775 Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue $ (938 ) $ (163 ) $ (775 ) Cash, cash equivalents and restricted cash, beginning of period $ 53,347 $ 53,347 $ — Cash, cash equivalents and restricted cash, end of period $ 96,750 $ 96,750 $ — For the Year Ended December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue $ (1,890 ) $ (252 ) $ (1,638 ) Cash, cash equivalents and restricted cash, beginning of period $ 53,347 $ 53,347 $ — Cash, cash equivalents and restricted cash, end of period $ 39,992 $ 39,992 $ — 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. Diluted net loss per share is computed using the if converted method. The Company allocates earnings first to preferred stockholders based on dividend rights and then to common and preferred stockholders based on ownership interests. 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, unvested shares of common stock and convertible preferred 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 ASU No. 2016-02, Leases (Topic 842), In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In December 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force), 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	6 Months Ended
	Jun. 30, 2019
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 June 30, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market mutual funds $ 54,989 $ — $ — $ 54,989 Repurchase agreements 60,000 — — 60,000 Asset-backed securities 16,968 11 — 16,979 Commercial paper 51,410 — — 51,410 Corporate debt securities 79,766 146 — 79,912 U.S. Treasury securities 31,993 29 — 32,022 Total $ 295,126 $ 186 $ — $ 295,312 As of December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market mutual funds $ 31,740 $ — $ — $ 31,740 Asset-backed securities 19,541 — (21 ) 19,520 Commercial paper 78,571 — — 78,571 Corporate debt securities 54,511 — (50 ) 54,461 U.S. Treasury securities 29,980 — (6 ) 29,974 Total $ 214,343 $ — $ (77 ) $ 214,266 The Company utilizes the specific identification method in computing realized gains and losses. The Company had no realized gains and losses on its available-for-sale securities for the three and six months ended June 30, 2019 and 2018. The contractual maturity dates of all of the Company’s investments are less than one year.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	4. FAIR VALUE MEASUREMENTS The Company’s financial instruments consist of cash and cash equivalents, short-term investments, restricted cash and accounts payable. The carrying amount of cash, restricted cash and accounts payable are each considered a reasonable estimate of fair value due to the short-term maturity. Assets measured at fair value on a recurring basis were as follows: Description June 30, 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 $ 54,989 $ 54,989 $ — $ — Repurchase agreements 60,000 — 60,000 — U.S. Treasury securities 93 93 — — Total cash equivalents $ 115,082 $ 55,082 $ 60,000 $ — Short-term investments: Asset-backed securities $ 16,979 $ — $ 16,979 $ — Commercial paper 51,410 — 51,410 — Corporate debt securities 79,912 — 79,912 — U.S. Treasury securities 31,929 — 31,929 — Total short-term investments $ 180,230 $ — $ 180,230 $ — Total financial assets $ 295,312 $ 55,082 $ 240,230 $ — Description December 31, 2018 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 $ 31,740 $ 31,740 $ — $ — Commercial paper 5,974 — 5,974 — U.S. Treasury securities 35 35 — — Total cash equivalents $ 37,749 $ 31,775 $ 5,974 $ — Short-term investments: Asset-backed securities $ 19,520 $ — $ 19,520 $ — Commercial paper 72,597 — 72,597 — Corporate debt securities 54,461 — 54,461 — U.S. Treasury securities 29,939 — 29,939 — Total short-term investments $ 176,517 $ — $ 176,517 $ — Total financial assets $ 214,266 $ 31,775 $ 182,491 $ — Short-term securities are valued using models or other valuation methodologies that use Level 2 inputs. These models are primarily industry-standard models that consider various assumptions, including time value, yield curve, volatility factors, default rates, current market and contractual prices for the underlying financial instruments, as well as other relevant economic measures. Substantially all of these assumptions are observable in the marketplace, can be derived from observable data or are supported by observable levels at which transactions are executed in the marketplace. There were no transfers between fair value measure levels during the three and six months ended June 30, 2019 and 2018.

Property and Equipment, Net

Property and Equipment, Net	6 Months Ended
	Jun. 30, 2019
Property Plant And Equipment [Abstract]
PROPERTY AND EQUIPMENT, NET	5. PROPERTY AND EQUIPMENT, NET Property and equipment, net consists of the following: As of June 30, 2019 December 31, 2018 (in thousands) Laboratory equipment $ 10,085 $ 9,309 Manufacturing equipment 4,523 — Computers and purchased software 521 248 Furniture and fixtures 1,386 1,066 Leasehold improvements 29,614 24,520 Assets not yet in service — 2,602 46,129 37,745 Less: accumulated depreciation and amortization (5,016 ) (2,108 ) Total property and equipment, net $ 41,113 $ 35,637 Depreciation expense for the three and six months ended June 30, 2019 was approximately $1.6 million and $2.9 million, respectively, compared to $0.2 million and $0.5 million, respectively, for the three and six months ended June 30, 2018. There were no disposals of property and equipment during the three and six months ended June 30, 2019 and 2018. Leasehold improvements consist primarily of costs associated with the buildout of the Company’s new research and development, manufacturing and general office space in Bedford, Massachusetts, which the Company occupied as of December 31, 2018. Assets not yet in service consists of manufacturing equipment and furniture purchased for the new space that was not yet in service.

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	6 Months Ended
	Jun. 30, 2019
Payables And Accruals [Abstract]
ACCRUED EXPENSES AND OTHER LIABILITIES	6. ACCRUED EXPENSES AND OTHER LIABILITIES Accrued expenses and other liabilities consist of the following: As of June 30, 2019 December 31, 2018 (in thousands) Accrued compensation and benefits $ 3,039 $ 3,370 Accrued research and development expenses 1,463 1,059 Accrued professional fees 440 195 Accrued unvested common stock subject to repurchase 76 112 Accrued other 288 304 Total accrued expenses and other liabilities $ 5,306 $ 5,040

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
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, with a related party. 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. The Company did not record a loss on the sublease as future payments to its landlord were not materially different from future rent payments expected from the subtenant over the term of the sublease. 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. Future minimum lease payments, net of anticipated sublease payments, as of June 30, 2019 are as follows: For the Years Ending December 31, Amount (in thousands) 2019 $ 1,379 2020 2,810 2021 2,815 2022 2,987 2023 3,077 Thereafter 10,366 Total future minimum lease payments $ 23,434 The lease agreement entered into in December 2017 allows for a tenant improvement allowance not to exceed $10.9 million to be applied to the total cost of tenant improvements to the leased premises. The tenant improvement allowance must be used on or before August 31, 2019 or it will be deemed forfeited with no further obligation by the landlord. Tenant improvement allowances due or received are recorded as deferred rent incentives on the Company’s condensed consolidated balance sheets and are recognized as a reduction to rent expense over the term of the lease. As of June 30, 2019, deferred rent incentives totaled $8.9 million. Rent expense, net of amortization of the deferred rent incentives, for the three and six months ended June 30, 2019 was $0.4 million and $0.6 million, respectively, as compared to $0.9 million and $1.8 million for the same periods in the prior year. The Company maintains letters of credit, secured by restricted cash, for security deposits totaling $1.3 million and $1.8 million as of June 30, 2019 and December 31, 2018, respectively, in conjunction with its current leases.

Stock Incentive Plans

Stock Incentive Plans	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
STOCK INCENTIVE PLANS	8. STOCK INCENTIVE PLANS 2015 Stock Incentive Plan In December 2015, the 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. In February 2017 and July 2017, the Board of Directors amended the 2015 Plan to increase the number of shares available for issuance under the 2015 Plan to 2,446,323 and 3,225,346, respectively. 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 June 30, 2019, 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, 2019, an additional 1,494,341 shares were added to the 2018 Plan, representing 4% of total common shares outstanding at December 31, 2018. At June 30, 2019, there were 2,666,267 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, 2019, an additional 373,585 shares were added to the 2018 ESPP, representing 1% of total common shares outstanding at December 31, 2018. At June 30, 2019, there were 699,254 shares available for future issuance under the 2018 ESPP. The Company commenced offerings under the 2018 ESPP on September 1, 2018. 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 with purchase periods ending 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 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 our common stock as of the first day of the offering period). During the six months ended June 30, 2019, 28,311 shares were issued under the 2018 ESPP for aggregate proceeds to the Company of $0.4 million. The Company recorded $0.1 million of stock-based compensation pursuant to the 2018 ESPP during the three and six months ended June 30, 2019. 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 Company recorded stock-based compensation expense related to stock options and shares purchased under the 2018 ESPP as follows: Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 (in thousands) Research and development $ 1,104 $ 167 $ 1,663 $ 300 General and administrative 865 402 1,550 649 $ 1,969 $ 569 $ 3,213 $ 949 As of June 30, 2019, there was $23.7 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 2.7 years. In determining the exercise prices for options granted, the Company’s Board of Directors considered the fair value of the common stock as of the measurement date. For the awards that were granted prior to the Company’s IPO, the Board of Directors determined the fair value of the common stock at each award grant date based upon a variety of factors, including the results obtained from an independent third-party valuation, the Company’s financial position and historical financial performance, the status of technological developments within the Company’s proposed products, an evaluation or benchmark of the Company’s competition, the current business climate in the marketplace, the illiquid nature of the common stock, arm’s length sales of the Company’s capital stock, including convertible preferred stock, the effect of the rights and preferences of the preferred shareholders, and the prospects of a liquidity event, among others. For awards granted after the Company’s IPO, the exercise price is equal to the closing price as quoted by Nasdaq on the grant date of the awards. The assumptions used in the Black-Scholes option pricing model for the three and six months ended June 30, 2019 and 2018 are as follows: Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Expected volatility 62.69% — 63.10% 56.36% 62.69% — 64.68% 52.80% - 60.12% Weighted-average risk-free interest rate 1.95% — 2.38% 2.84% 1.95% — 2.60% 2.33% - 2.84% Expected dividend yield — % — % — % — % Expected term (in years) 6.25 6.25 6.25 5.5 - 7.6 Underlying common stock fair value $19.51 — $30.34 $20.98 $19.51 — $30.34 $6.63 - $20.98 A summary of option activity under the Plans during the six months ended June 30, 2019 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, 2019 3,203,122 $ 10.47 8.9 $ 39,018 Granted 760,355 $ 25.02 Exercised (31,280 ) $ 2.43 Cancelled/Forfeited (35,355 ) $ 18.16 Outstanding at June 30, 2019 3,896,842 $ 13.31 8.5 $ 30,939 Vested and expected to vest at June 30, 2019 3,896,842 $ 13.31 8.5 $ 30,939 Exercisable at June 30, 2019 1,229,412 $ 7.36 7.8 $ 15,531 The total intrinsic value of options exercised during the six months ended June 30, 2019 and 2018 was $0.6 million and $1.0 million, respectively. The weighted-average grant date fair value per share of options granted during the six months ended June 30, 2019 and 2018 was $15.12 and $8.38, 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, 2019 151,289 Vested (62,608 ) Issued — Repurchased — Unvested shares - June 30, 2019 88,681 As of June 30, 2019 and December 31, 2018, 88,681 and 151,289 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 $0.1 million as of each date.

Net Loss Per Share

Net Loss Per Share	6 Months Ended
	Jun. 30, 2019
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 June 30, 2019 and 2018, from the computation of diluted net loss per share attributable to common stockholders because including them would have had an anti-dilutive effect: As of June 30, 2019 2018 Unvested common stock from early exercise of options 88,681 219,747 Stock options to purchase common stock 3,896,842 2,641,800 Total 3,985,523 2,861,547

Collaboration and License Agree

Collaboration and License Agreement	6 Months Ended
	Jun. 30, 2019
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 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 our 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 June 30, 2019, the aggregate amount of the transaction price related to the unsatisfied portion of the performance obligation is $31.3 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. In February 2019, Novartis elected to discontinue the sickle cell disease program under the Collaboration Agreement. Future costs associated with this target were removed from the estimated total costs in the cost-to-cost model, which resulted in an insignificant adjustment to revenue during the period. During the three and six months ended June 30, 2019, the Company recognized revenue under the Collaboration Agreement of $0.4 million and $0.7 million, respectively, of which $0.1 million and $0.2 million was included in deferred revenue at the beginning of each such period. During the three and six months ended June 30, 2018, the Company recognized revenue under the Collaboration Agreement of $1.4 million and $2.5 million, respectively, of which $0.5 million and $0.8 million was included in deferred revenue at the beginning of each such period. As of June 30, 2019 and December 31, 2018, there was approximately $31.3 million and $31.5 million of deferred revenue related to the Collaboration Agreement, respectively. In addition, as of June 30, 2019 and December 31, 2018, the Company has recorded accounts receivable of $0.3 million and $0.8 million, respectively, related to reimbursable research and development costs under the Collaboration Agreement, which are included in prepaid expenses and other current assets on the condensed consolidated balance sheets.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
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, 2018, included in the Company's Annual Report on Form 10-K on file with the SEC. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position as of June 30, 2019, and consolidated results of operations for the three and six months ended June 30, 2019 and 2018, and cash flows for the six months ended June 30, 2019 and 2018. Such adjustments are of a normal and recurring nature. The results of operations for the three and six months ended June 30, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019.
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, useful lives assigned to property and equipment, as well as the fair values of common stock and convertible preferred stock. The Company assesses estimates on an ongoing basis; however, actual results could materially differ from those estimates.
Comprehensive Income (Loss)	Comprehensive Income (Loss) —Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. The Company’s only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments.
Cash and Cash Equivalents and Restricted Cash	Cash and Cash Equivalents and Restricted Cash —Cash and cash equivalents consist of standard checking accounts, money market accounts and certain investments. The Company considers all highly liquid investments with original or remaining maturities at the time of purchase of 90 days or less to be cash equivalents. Restricted cash consists of cash serving as collateral for letters of credit issued for security deposits for the Company’s facility leases in Bedford, Massachusetts. The following table provides a reconciliation of cash, cash equivalents and restricted cash to amounts shown in the condensed consolidated statements of cash flows: June 30, 2019 2018 (in thousands) Cash and cash equivalents $ 115,566 $ 94,960 Restricted cash 1,274 1,790 Total cash, cash equivalents and restricted cash $ 116,840 $ 96,750
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 (deficit) 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 (expense).
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— In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue (Topic 606): Revenue from Contracts with Customers (“ASU 2014-09”), which amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in FASB ASC Topic 605, Revenue Recognition (“ASC 605”), and creates a new 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, an entity 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 good 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 good 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. The Company adopted ASC 606 effective January 1, 2019, using the full retrospective transition method. Under this method, the Company revised its condensed consolidated financial statements for prior period amounts as if ASC 606 had been effective for such periods. The references "as revised" used herein refer to revisions of data for the three and six months ended June 30, 2019 and the year ended December 31, 2018 as a result of the adoption of ASC 606. The adoption of ASC 606 did not have an impact on periods prior to January 1, 2018. In addition to revisions to financial statement amounts and related tables, Note 10 has been revised to reflect the adoption of ASC 606. As part of the adoption, the Company reviewed its collaboration and license agreement with Novartis to determine the cumulative effect impact related to the adoption of ASC 606. (For a complete discussion of the accounting for the Company’s agreement with Novartis, see Note 10.) The adoption of ASC 606 resulted in a change to the pattern of revenue recognition whereby the Company expects to recognize revenue from its collaboration agreement with Novartis as costs are incurred, which likely will not occur evenly over the performance period as a result of applying the cost-to-cost method, in contrast to recognizing revenue on a straight-line basis over the estimated performance period under ASC 605. The impact on previously reported amounts as a result of the adoption of ASC 606 is as follows: Condensed Consolidated Balance Sheets December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Current portion of deferred revenue $ 770 $ 3,684 $ (2,914 ) Long-term portion of deferred revenue $ 30,750 $ 29,474 $ 1,276 Accumulated deficit $ (95,758 ) $ (97,396 ) $ 1,638 Condensed Consolidated Statements of Operations and Comprehensive Loss Three Months Ended June 30, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 1,395 $ 926 $ 469 Loss from operations $ (13,422 ) $ (13,891 ) $ 469 Net loss $ (12,426 ) $ (12,895 ) $ 469 Net loss per share attributable to common stockholders-basic and diluted $ (0.34 ) $ (0.35 ) $ 0.01 Six Months Ended June 30, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 2,524 $ 1,749 $ 775 Loss from operations $ (24,119 ) $ (24,894 ) $ 775 Net loss $ (22,643 ) $ (23,418 ) $ 775 Net loss per share attributable to common stockholders-basic and diluted $ (1.15 ) $ (1.19 ) $ 0.04 For the Year Ended December 31, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 5,322 $ 3,684 $ 1,638 Loss from operations $ (59,926 ) $ (61,564 ) $ 1,638 Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Net loss per share attributable to common stockholders-basic and diluted $ (1.95 ) $ (2.00 ) $ 0.05 Condensed Consolidated Statement of Cash Flows Six Months Ended June 30, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Net loss $ (22,643 ) $ (23,418 ) $ 775 Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue $ (938 ) $ (163 ) $ (775 ) Cash, cash equivalents and restricted cash, beginning of period $ 53,347 $ 53,347 $ — Cash, cash equivalents and restricted cash, end of period $ 96,750 $ 96,750 $ — For the Year Ended December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue $ (1,890 ) $ (252 ) $ (1,638 ) Cash, cash equivalents and restricted cash, beginning of period $ 53,347 $ 53,347 $ — Cash, cash equivalents and restricted cash, end of period $ 39,992 $ 39,992 $ —
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. Diluted net loss per share is computed using the if converted method. The Company allocates earnings first to preferred stockholders based on dividend rights and then to common and preferred stockholders based on ownership interests. 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, unvested shares of common stock and convertible preferred 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 ASU No. 2016-02, Leases (Topic 842), In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In December 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force), 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)	6 Months Ended
	Jun. 30, 2019
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash to amounts shown in the condensed consolidated statements of cash flows: June 30, 2019 2018 (in thousands) Cash and cash equivalents $ 115,566 $ 94,960 Restricted cash 1,274 1,790 Total cash, cash equivalents and restricted cash $ 116,840 $ 96,750
ASC 606
Impact of Adoption of ASC 606 on Previously Reported Amounts of Financial Statement	As part of the adoption, the Company reviewed its collaboration and license agreement with Novartis to determine the cumulative effect impact related to the adoption of ASC 606. (For a complete discussion of the accounting for the Company’s agreement with Novartis, see Note 10.) The adoption of ASC 606 resulted in a change to the pattern of revenue recognition whereby the Company expects to recognize revenue from its collaboration agreement with Novartis as costs are incurred, which likely will not occur evenly over the performance period as a result of applying the cost-to-cost method, in contrast to recognizing revenue on a straight-line basis over the estimated performance period under ASC 605. The impact on previously reported amounts as a result of the adoption of ASC 606 is as follows: Condensed Consolidated Balance Sheets December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Current portion of deferred revenue $ 770 $ 3,684 $ (2,914 ) Long-term portion of deferred revenue $ 30,750 $ 29,474 $ 1,276 Accumulated deficit $ (95,758 ) $ (97,396 ) $ 1,638 Condensed Consolidated Statements of Operations and Comprehensive Loss Three Months Ended June 30, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 1,395 $ 926 $ 469 Loss from operations $ (13,422 ) $ (13,891 ) $ 469 Net loss $ (12,426 ) $ (12,895 ) $ 469 Net loss per share attributable to common stockholders-basic and diluted $ (0.34 ) $ (0.35 ) $ 0.01 Six Months Ended June 30, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 2,524 $ 1,749 $ 775 Loss from operations $ (24,119 ) $ (24,894 ) $ 775 Net loss $ (22,643 ) $ (23,418 ) $ 775 Net loss per share attributable to common stockholders-basic and diluted $ (1.15 ) $ (1.19 ) $ 0.04 For the Year Ended December 31, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 5,322 $ 3,684 $ 1,638 Loss from operations $ (59,926 ) $ (61,564 ) $ 1,638 Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Net loss per share attributable to common stockholders-basic and diluted $ (1.95 ) $ (2.00 ) $ 0.05 Condensed Consolidated Statement of Cash Flows Six Months Ended June 30, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Net loss $ (22,643 ) $ (23,418 ) $ 775 Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue $ (938 ) $ (163 ) $ (775 ) Cash, cash equivalents and restricted cash, beginning of period $ 53,347 $ 53,347 $ — Cash, cash equivalents and restricted cash, end of period $ 96,750 $ 96,750 $ — For the Year Ended December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue $ (1,890 ) $ (252 ) $ (1,638 ) Cash, cash equivalents and restricted cash, beginning of period $ 53,347 $ 53,347 $ — Cash, cash equivalents and restricted cash, end of period $ 39,992 $ 39,992 $ —

Short-Term Investments (Tables)

Short-Term Investments (Tables)	6 Months Ended
	Jun. 30, 2019
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 June 30, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market mutual funds $ 54,989 $ — $ — $ 54,989 Repurchase agreements 60,000 — — 60,000 Asset-backed securities 16,968 11 — 16,979 Commercial paper 51,410 — — 51,410 Corporate debt securities 79,766 146 — 79,912 U.S. Treasury securities 31,993 29 — 32,022 Total $ 295,126 $ 186 $ — $ 295,312 As of December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market mutual funds $ 31,740 $ — $ — $ 31,740 Asset-backed securities 19,541 — (21 ) 19,520 Commercial paper 78,571 — — 78,571 Corporate debt securities 54,511 — (50 ) 54,461 U.S. Treasury securities 29,980 — (6 ) 29,974 Total $ 214,343 $ — $ (77 ) $ 214,266

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on Recurring Basis	Assets measured at fair value on a recurring basis were as follows: Description June 30, 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 $ 54,989 $ 54,989 $ — $ — Repurchase agreements 60,000 — 60,000 — U.S. Treasury securities 93 93 — — Total cash equivalents $ 115,082 $ 55,082 $ 60,000 $ — Short-term investments: Asset-backed securities $ 16,979 $ — $ 16,979 $ — Commercial paper 51,410 — 51,410 — Corporate debt securities 79,912 — 79,912 — U.S. Treasury securities 31,929 — 31,929 — Total short-term investments $ 180,230 $ — $ 180,230 $ — Total financial assets $ 295,312 $ 55,082 $ 240,230 $ — Description December 31, 2018 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 $ 31,740 $ 31,740 $ — $ — Commercial paper 5,974 — 5,974 — U.S. Treasury securities 35 35 — — Total cash equivalents $ 37,749 $ 31,775 $ 5,974 $ — Short-term investments: Asset-backed securities $ 19,520 $ — $ 19,520 $ — Commercial paper 72,597 — 72,597 — Corporate debt securities 54,461 — 54,461 — U.S. Treasury securities 29,939 — 29,939 — Total short-term investments $ 176,517 $ — $ 176,517 $ — Total financial assets $ 214,266 $ 31,775 $ 182,491 $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	6 Months Ended
	Jun. 30, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consists of the following: As of June 30, 2019 December 31, 2018 (in thousands) Laboratory equipment $ 10,085 $ 9,309 Manufacturing equipment 4,523 — Computers and purchased software 521 248 Furniture and fixtures 1,386 1,066 Leasehold improvements 29,614 24,520 Assets not yet in service — 2,602 46,129 37,745 Less: accumulated depreciation and amortization (5,016 ) (2,108 ) Total property and equipment, net $ 41,113 $ 35,637

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	6 Months Ended
	Jun. 30, 2019
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consist of the following: As of June 30, 2019 December 31, 2018 (in thousands) Accrued compensation and benefits $ 3,039 $ 3,370 Accrued research and development expenses 1,463 1,059 Accrued professional fees 440 195 Accrued unvested common stock subject to repurchase 76 112 Accrued other 288 304 Total accrued expenses and other liabilities $ 5,306 $ 5,040

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments Net of Anticipated Sublease Payments	Future minimum lease payments, net of anticipated sublease payments, as of June 30, 2019 are as follows: For the Years Ending December 31, Amount (in thousands) 2019 $ 1,379 2020 2,810 2021 2,815 2022 2,987 2023 3,077 Thereafter 10,366 Total future minimum lease payments $ 23,434

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
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 June 30, Six months ended June 30, 2019 2018 2019 2018 (in thousands) Research and development $ 1,104 $ 167 $ 1,663 $ 300 General and administrative 865 402 1,550 649 $ 1,969 $ 569 $ 3,213 $ 949
Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model	The assumptions used in the Black-Scholes option pricing model for the three and six months ended June 30, 2019 and 2018 are as follows: Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Expected volatility 62.69% — 63.10% 56.36% 62.69% — 64.68% 52.80% - 60.12% Weighted-average risk-free interest rate 1.95% — 2.38% 2.84% 1.95% — 2.60% 2.33% - 2.84% Expected dividend yield — % — % — % — % Expected term (in years) 6.25 6.25 6.25 5.5 - 7.6 Underlying common stock fair value $19.51 — $30.34 $20.98 $19.51 — $30.34 $6.63 - $20.98
Summary of Option Activity under Plans	A summary of option activity under the Plans during the six months ended June 30, 2019 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, 2019 3,203,122 $ 10.47 8.9 $ 39,018 Granted 760,355 $ 25.02 Exercised (31,280 ) $ 2.43 Cancelled/Forfeited (35,355 ) $ 18.16 Outstanding at June 30, 2019 3,896,842 $ 13.31 8.5 $ 30,939 Vested and expected to vest at June 30, 2019 3,896,842 $ 13.31 8.5 $ 30,939 Exercisable at June 30, 2019 1,229,412 $ 7.36 7.8 $ 15,531
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, 2019 151,289 Vested (62,608 ) Issued — Repurchased — Unvested shares - June 30, 2019 88,681

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Schedule of Anti-dilutive Effect Excluded from Computation of Diluted Net Loss Per Share Attributable to Common Stockholders	The Company excluded the following potential common shares, presented based on amounts outstanding at June 30, 2019 and 2018, from the computation of diluted net loss per share attributable to common stockholders because including them would have had an anti-dilutive effect: As of June 30, 2019 2018 Unvested common stock from early exercise of options 88,681 219,747 Stock options to purchase common stock 3,896,842 2,641,800 Total 3,985,523 2,861,547

Nature of Business and Basis _2

Nature of Business and Basis of Presentation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Apr. 26, 2019	Apr. 12, 2019	Apr. 01, 2019	Apr. 02, 2018	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from initial public offering after underwriting discounts and commissions and offering expenses										$ 150,843
Conversion of convertible preferred stock into common stock upon initial public offering, Shares				24,168,656
Common stock, shares issued					43,958,295				43,958,295		37,509,815
Net loss					$ 26,270	$ 23,852	$ 12,426	$ 10,217	$ 50,122	$ 22,643	$ 55,577
Accumulated deficit					$ 145,880				$ 145,880		$ 95,758
Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold							10,350,000
Conversion of convertible preferred stock into common stock upon initial public offering, Shares							24,168,656
Sales Agreement
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Common stock, shares issued					0				0
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
Common Stock, Preferred Stock, Debt Securities, Warrants and Units | Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings			$ 350,000
At-The-Market Sales Agreement | Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings			$ 100,000
Follow-on Public 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	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 115,566	$ 38,220	$ 94,960
Restricted cash	1,274	$ 1,772	1,790
Total cash, cash equivalents and restricted cash	$ 116,840		$ 96,750

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 on Previously Reported Amounts of Condensed Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current portion of deferred revenue	$ 690	$ 770
Long-term portion of deferred revenue	30,597	30,750
Accumulated deficit	$ (145,880)	(95,758)
ASC 606 | As Originally Reported Under ASC 605
Current portion of deferred revenue		3,684
Long-term portion of deferred revenue		29,474
Accumulated deficit		(97,396)
ASC 606 | Effect of Change
Current portion of deferred revenue		(2,914)
Long-term portion of deferred revenue		1,276
Accumulated deficit		$ 1,638

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 on Previously Reported Amounts of Condensed Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Collaboration revenue	$ 392		$ 1,395		$ 662	$ 2,524	$ 5,322
Loss from operations	(27,973)		(13,422)		(53,096)	(24,119)	(59,926)
Net loss	$ (26,270)	$ (23,852)	$ (12,426)	$ (10,217)	$ (50,122)	$ (22,643)	$ (55,577)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.61)		$ (0.34)		$ (1.25)	$ (1.15)	$ (1.95)
ASC 606 | As Originally Reported Under ASC 605
Collaboration revenue			$ 926			$ 1,749	$ 3,684
Loss from operations			(13,891)			(24,894)	(61,564)
Net loss			$ (12,895)			$ (23,418)	$ (57,215)
Net loss per share attributable to common stockholders-basic and diluted			$ (0.35)			$ (1.19)	$ (2)
ASC 606 | Effect of Change
Collaboration revenue			$ 469			$ 775	$ 1,638
Loss from operations			469			775	1,638
Net loss			$ 469			$ 775	$ 1,638
Net loss per share attributable to common stockholders-basic and diluted			$ 0.01			$ 0.04	$ 0.05

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 on Previously Reported Amounts of Condensed Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Net loss	$ (50,122)	$ (22,643)	$ (55,577)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue	(233)	(938)	(1,890)
Cash, cash equivalents and restricted cash, beginning of period	39,992	53,347	53,347
Cash, cash equivalents and restricted cash, end of period	116,840	96,750	39,992
ASC 606 | As Originally Reported Under ASC 605
Net loss		(23,418)	(57,215)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue		(163)	(252)
Cash, cash equivalents and restricted cash, beginning of period	$ 39,992	53,347	53,347
Cash, cash equivalents and restricted cash, end of period		96,750	39,992
ASC 606 | Effect of Change
Net loss		775	1,638
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue		$ (775)	$ (1,638)

Short-Term Investments - Summar

Short-Term Investments - Summary of Investments Included in Cash Equivalents and Short Term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	$ 295,126	$ 214,343
Cash equivalents and short-term investments, Unrealized Gains	186
Cash equivalents and short-term investments, Unrealized Losses		(77)
Cash equivalents and short-term investments, Fair Value	295,312	214,266
Money Market Mutual Funds
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	54,989	31,740
Cash equivalents and short-term investments, Fair Value	54,989	31,740
Repurchase Agreements
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	60,000
Cash equivalents and short-term investments, Fair Value	60,000
Asset-backed Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	16,968	19,541
Cash equivalents and short-term investments, Unrealized Gains	11
Cash equivalents and short-term investments, Unrealized Losses		(21)
Cash equivalents and short-term investments, Fair Value	16,979	19,520
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	51,410	78,571
Cash equivalents and short-term investments, Fair Value	51,410	78,571
Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	79,766	54,511
Cash equivalents and short-term investments, Unrealized Gains	146
Cash equivalents and short-term investments, Unrealized Losses		(50)
Cash equivalents and short-term investments, Fair Value	79,912	54,461
U.S Treasury Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	31,993	29,980
Cash equivalents and short-term investments, Unrealized Gains	29
Cash equivalents and short-term investments, Unrealized Losses		(6)
Cash equivalents and short-term investments, Fair Value	$ 32,022	$ 29,974

Short-Term Investments - Additi

Short-Term Investments - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Cash Cash Equivalents And Available For Sale Securities [Abstract]
Realized gains and losses on available-for-sale securities	$ 0	$ 0	$ 0	$ 0
Contractual maturity date of investments			Less than one year

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	$ 180,230	$ 176,517
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	115,082	37,749
Short-term investments, fair value	180,230	176,517
Financial assets, fair value	295,312	214,266
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	54,989	31,740
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	16,979	19,520
Fair Value, Measurements, Recurring | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value		5,974
Short-term investments, fair value	51,410	72,597
Fair Value, Measurements, Recurring | U.S Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	93	35
Short-term investments, fair value	31,929	29,939
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	79,912	54,461
Fair Value, Measurements, Recurring | Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	60,000
Fair Value, Measurements, Recurring | Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	55,082	31,775
Financial assets, fair value	55,082	31,775
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	54,989	31,740
Fair Value, Measurements, Recurring | Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) | U.S Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	93	35
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	60,000	5,974
Short-term investments, fair value	180,230	176,517
Financial assets, fair value	240,230	182,491
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	16,979	19,520
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]
Cash equivalents, fair value		5,974
Short-term investments, fair value	51,410	72,597
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	31,929	29,939
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	79,912	$ 54,461
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	$ 60,000

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Transfers between fair value measure levels	$ 0	$ 0	$ 0	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 46,129	$ 37,745
Less: accumulated depreciation and amortization	(5,016)	(2,108)
Property and equipment, net	41,113	35,637
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	10,085	9,309
Manufacturing Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,523
Computers and Purchased Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	521	248
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,386	1,066
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 29,614	24,520
Assets Not Yet in Service
Property Plant And Equipment [Line Items]
Property and equipment, gross		$ 2,602

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Property Plant And Equipment [Abstract]
Depreciation	$ 1,600,000	$ 200,000	$ 2,908,000	$ 458,000
Disposal of property and equipment	$ 0	$ 0	$ 0	$ 0

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued compensation and benefits	$ 3,039	$ 3,370
Accrued research and development expenses	1,463	1,059
Accrued professional fees	440	195
Accrued unvested common stock subject to repurchase	76	112
Accrued other	288	304
Total accrued expenses and other liabilities	$ 5,306	$ 5,040

Commitment and Contingencies -

Commitment and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended		6 Months Ended
	Dec. 31, 2017USD ($)ft²Phase	Sep. 30, 2016	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Commitment And Contingencies [Line Items]
Deferred rent incentives			$ 8,900,000		$ 8,900,000
Rent expense, net			400,000	$ 900,000	600,000	$ 1,800,000
Letters Of Credit And Secured By Restricted Cash
Commitment And Contingencies [Line Items]
Security deposit			1,300,000		1,300,000		$ 1,800,000
Maximum
Commitment And Contingencies [Line Items]
Tenant improvement allowance			10,900,000		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		$ 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 Start Date Year And Month	2018-09
Bedford, Massachusetts | Phase Two
Commitment And Contingencies [Line Items]
Office space leased | ft²	21,000
Rent Start Date Year And Month	2019-03

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Jun. 30, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 1,379
2020	2,810
2021	2,815
2022	2,987
2023	3,077
Thereafter	10,366
Total future minimum lease payments	$ 23,434

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2019	Sep. 01, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Jul. 31, 2017	Feb. 28, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate proceeds from shares issued under the plan						$ 396
Stock-based compensation				$ 1,969	$ 569	3,213	$ 949
Total intrinsic value of options exercised						$ 600	$ 1,000
Weighted-average grant date fair value per share for options granted						$ 15.12	$ 8.38
Remaining shares subject to repurchase				88,681		88,681		151,289
Liability for unvested common shares subject to repurchase included in accrued expenses and other liabilities				$ 76		$ 76		$ 112
2015 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance									3,225,346	2,446,323
Stock option vesting period						4 years
Stock options expiration period						10 years
Number of additional shares available for future grant				0		0
2018 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance			3,186,205
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock	4.00%		4.00%
Maximum shares of common stock may be issued			20,887,347
Number of additional shares added to the plan	1,494,341
Number of shares outstanding available for future grant				2,666,267		2,666,267
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	373,585
Number of shares outstanding available for future grant				699,254		699,254
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						28,311
Aggregate proceeds from shares issued under the plan						$ 400
Stock-based compensation				$ 100		100
2015 and 2018 Stock Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense				$ 23,700		$ 23,700
Unrecognized compensation expense estimated to be recognized over period						2 years 8 months 12 days

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 1,969	$ 569	$ 3,213	$ 949
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	1,104	167	1,663	300
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 865	$ 402	$ 1,550	$ 649

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model (Details) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility		56.36%
Expected volatility, minimum	62.69%		62.69%	52.80%
Expected volatility, maximum	63.10%		64.68%	60.12%
Weighted-average risk-free interest rate		2.84%
Weighted-average risk-free interest rate, minimum	1.95%		1.95%	2.33%
Weighted-average risk-free interest rate, maximum	2.38%		2.60%	2.84%
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months
Underlying common stock fair value		$ 20.98
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)				5 years 6 months
Underlying common stock fair value	$ 19.51		$ 19.51	$ 6.63
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)				7 years 7 months 6 days
Underlying common stock fair value	$ 30.34		$ 30.34	$ 20.98

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Option Activity under Plans (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding at Beginning Balance | shares	3,203,122
Number of Options, Granted | shares	760,355
Number of Options, Exercised | shares	(31,280)
Number of Options, Cancelled/Forfeited | shares	(35,355)
Number of Options, Outstanding at Ending Balance | shares	3,896,842	3,203,122
Number of Options, Vested and Expected to vest at June 30, 2019 | shares	3,896,842
Number of Options, Exercisable at June 30, 2019 | shares	1,229,412
Weighted-Average Exercise Price per Share, Outstanding at Beginning Balance | $ / shares	$ 10.47
Weighted-Average Exercise Price per Share, Granted | $ / shares	25.02
Weighted-Average Exercise Price per Share, Exercised | $ / shares	2.43
Weighted-Average Exercise Price per Share, Cancelled/Forfeited | $ / shares	18.16
Weighted-Average Exercise Price per Share, Outstanding at Ending Balance | $ / shares	13.31	$ 10.47
Weighted-Average Exercise Price per Share, Vested and Expected to vest at June 30, 2019 | $ / shares	13.31
Weighted-Average Exercise Price Per Share, Exercisable at June 30, 2019 | $ / shares	$ 7.36
Weighted Average Remaining Contractual Term, Outstanding	8 years 6 months	8 years 10 months 24 days
Weighted Average Remaining Contractual Term, Vested and Expected to vest at June 30, 2019	8 years 6 months
Weighted Average Remaining Contractual Term, Exercisable at June 30, 2019	7 years 9 months 18 days
Aggregate Intrinsic Value, Outstanding Ending Balance | $	$ 30,939	$ 39,018
Aggregate Intrinsic Value, Vested and Expected to vest at June 30, 2019 | $	30,939
Aggregate Intrinsic Value, Exercisable at June 30, 2019 | $	$ 15,531

Stock Incentive Plans - Summa_2

Stock Incentive Plans - Summary of Unvested Common Stock from Early Exercises Subject to Repurchase (Details)	6 Months Ended
	Jun. 30, 2019shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unvested shares—Beginning Balance	151,289
Vested	(62,608)
Unvested shares—Ending Balance	88,681

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Anti-dilutive Effect Excluded from Computation of Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	3,985,523	2,861,547
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	88,681	219,747
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	3,896,842	2,641,800

Collaboration and License Agr_2

Collaboration and License Agreement - Additional Information (Details)	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Nov. 30, 2017USD ($)TargetCandidateorProduct	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Apr. 01, 2019USD ($)	Jan. 01, 2019USD ($)	Apr. 01, 2018USD ($)	Jan. 01, 2018USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement revenue recognized		$ 392,000	$ 1,395,000	$ 662,000	$ 2,524,000	$ 5,322,000
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		400,000	$ 1,400,000	700,000	$ 2,500,000
Deferred revenue		31,300,000		31,300,000		31,500,000	$ 100,000	$ 200,000	$ 500,000	$ 800,000
Novartis | Collaborative Arrangement | Reimbursable Research and Development Costs
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable		$ 300,000		300,000		$ 800,000
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	Jun. 30, 2019USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2017-11-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
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	$ 31.3