Telesis Bio 10-Q 31 Jul 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Jul. 29, 2019
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	TBIO
Entity Registrant Name	Translate Bio, Inc.
Entity Central Index Key	0001693415
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Address, State or Province	DE
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common stock
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		51,010,368

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 74,904	$ 55,199
Short-term investments	71,961	88,904
Prepaid expenses and other current assets	6,069	4,474
Restricted cash	950	1,025
Total current assets	153,884	149,602
Property and equipment, net	10,903	10,245
Right-of-use assets, net	10,650
Goodwill	21,359	21,359
Intangible assets, net	105,630	106,445
Deferred offering costs	123
Total assets	302,549	287,651
Current liabilities:
Accounts payable	3,347	5,168
Accrued expenses	7,246	6,547
Current portion of deferred revenue	9,015	2,572
Current portion of operating lease liability	449
Total current liabilities	20,057	14,287
Long-term portion of contingent consideration	120,233	103,642
Deferred revenue, net of current portion	34,192	41,841
Deferred tax liabilities		481
Deferred rent		2,105
Operating lease liability, net of current portion	12,370
Total liabilities	186,852	162,356
Commitments and contingencies (Notes 3, 4 and 13)
Stockholders' equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 shares authorized as of June 30, 2019 and December 31, 2018; no shares issued and outstanding as of June 30, 2019 and December 31, 2018
Common stock, $0.001 par value; 200,000,000 shares authorized as of June 30, 2019 and December 31, 2018; 51,010,368 shares and 45,139,955 shares issued and outstanding as of June 30, 2019 and December 31, 2018, respectively	51	45
Additional paid-in capital	422,309	371,257
Accumulated deficit	(307,233)	(246,203)
Accumulated other comprehensive income	570	196
Total stockholders' equity (deficit)	115,697	125,295
Total liabilities and stockholders' equity (deficit)	$ 302,549	$ 287,651

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
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	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	51,010,368	45,139,955
Common stock, shares outstanding	51,010,368	45,139,955

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Collaboration revenue	$ 1,174		$ 2,648
Operating expenses:
Research and development	16,625	$ 15,219	34,048	$ 27,921
General and administrative	7,850	5,991	14,403	10,769
Change in fair value of contingent consideration	4,889	7,852	16,591	12,760
Total operating expenses	29,364	29,062	65,042	51,450
Loss from operations	(28,190)	(29,062)	(62,394)	(51,450)
Other income (expense):
Interest income	358	91	878	181
Other expense		(32)		(45)
Total other income (expense), net	358	59	878	136
Loss before benefit from income taxes	(27,832)	(29,003)	(61,516)	(51,314)
Benefit from income taxes		1,500	486	2,602
Net loss	(27,832)	(27,503)	(61,030)	(48,712)
Accretion of redeemable convertible preferred stock to redemption value		(459)		(644)
Net loss attributable to common stockholders	$ (27,832)	$ (27,962)	$ (61,030)	$ (49,356)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.57)	$ (3.04)	$ (1.30)	$ (5.40)
Weighted average common shares outstanding—basic and diluted	48,749,627	9,187,207	46,866,842	9,139,638

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (27,832)	$ (27,503)	$ (61,030)	$ (48,712)
Other comprehensive income (loss):
Unrealized gains (losses) on available-for-sale securities, net of tax of $0	219		374	(79)
Comprehensive loss	$ (27,613)	$ (27,503)	$ (60,656)	$ (48,791)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Unrealized gains (losses) on available-for-sale securities, tax	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income [Member]	Redeemable Convertible Preferred Stock [Member]
Beginning balances at Dec. 31, 2017	$ (93,515)	$ 10	$ 55,204	$ (148,808)	$ 79
Beginning balance, Shares at Dec. 31, 2017		9,582,791
Redeemable Convertible Preferred Stock, beginning balance at Dec. 31, 2017						$ 192,896
Redeemable convertible preferred stock, shares outstanding, beginning balance at Dec. 31, 2017						142,288,292
Accretion of redeemable convertible preferred stock to redemption value	(185)		(185)			$ 185
Unrealized gains on available-for-sale securities	(79)				(79)
Stock-based compensation expense	1,383		1,383
Net loss	(21,209)			(21,209)
Redeemable convertible preferred stock, shares outstanding, ending balance at Mar. 31, 2018						142,288,292
Redeemable Convertible Preferred Stock, ending balance at Mar. 31, 2018						$ 193,081
Ending balances at Mar. 31, 2018	(113,605)	$ 10	56,402	(170,017)
Ending balance, shares at Mar. 31, 2018		9,582,791
Beginning balances at Dec. 31, 2017	(93,515)	$ 10	55,204	(148,808)	79
Beginning balance, Shares at Dec. 31, 2017		9,582,791
Redeemable Convertible Preferred Stock, beginning balance at Dec. 31, 2017						$ 192,896
Redeemable convertible preferred stock, shares outstanding, beginning balance at Dec. 31, 2017						142,288,292
Accretion of redeemable convertible preferred stock to redemption value	(644)
Unrealized gains on available-for-sale securities	(79)
Net loss	(48,712)
Redeemable convertible preferred stock, shares outstanding, ending balance at Jun. 30, 2018						142,288,292
Redeemable Convertible Preferred Stock, ending balance at Jun. 30, 2018						$ 193,540
Ending balances at Jun. 30, 2018	(138,414)	$ 10	59,096	(197,520)
Ending balance, shares at Jun. 30, 2018		9,631,939
Beginning balances at Mar. 31, 2018	(113,605)	$ 10	56,402	(170,017)
Beginning balance, Shares at Mar. 31, 2018		9,582,791
Redeemable Convertible Preferred Stock, beginning balance at Mar. 31, 2018						$ 193,081
Redeemable convertible preferred stock, shares outstanding, beginning balance at Mar. 31, 2018						142,288,292
Forfeited restricted common stock, Shares		(311)
Exercise of stock options	278		278
Exercise of stock options, Shares		49,459
Accretion of redeemable convertible preferred stock to redemption value	(459)		(459)			$ 459
Stock-based compensation expense	2,875		2,875
Net loss	(27,503)			(27,503)
Redeemable convertible preferred stock, shares outstanding, ending balance at Jun. 30, 2018						142,288,292
Redeemable Convertible Preferred Stock, ending balance at Jun. 30, 2018						$ 193,540
Ending balances at Jun. 30, 2018	(138,414)	$ 10	59,096	(197,520)
Ending balance, shares at Jun. 30, 2018		9,631,939
Beginning balances at Dec. 31, 2018	$ 125,295	$ 45	371,257	(246,203)	196
Beginning balance, Shares at Dec. 31, 2018		45,139,955
Redeemable convertible preferred stock, shares outstanding, beginning balance at Dec. 31, 2018	0
Exercise of stock options	$ 897		897
Exercise of stock options, Shares		154,484
Unrealized gains on available-for-sale securities	155				155
Stock-based compensation expense	1,959		1,959
Net loss	(33,198)			(33,198)
Ending balances at Mar. 31, 2019	95,108	$ 45	374,113	(279,401)	351
Ending balance, shares at Mar. 31, 2019		45,294,439
Beginning balances at Dec. 31, 2018	$ 125,295	$ 45	371,257	(246,203)	196
Beginning balance, Shares at Dec. 31, 2018		45,139,955
Redeemable convertible preferred stock, shares outstanding, beginning balance at Dec. 31, 2018	0
Issuance of common stock in connection with a former employee letter agreement (Note 9)	$ 847
Unrealized gains on available-for-sale securities	374
Net loss	$ (61,030)
Redeemable convertible preferred stock, shares outstanding, ending balance at Jun. 30, 2019	0
Ending balances at Jun. 30, 2019	$ 115,697	$ 51	422,309	(307,233)	570
Ending balance, shares at Jun. 30, 2019		51,010,368
Beginning balances at Mar. 31, 2019	95,108	$ 45	374,113	(279,401)	351
Beginning balance, Shares at Mar. 31, 2019		45,294,439
Issuance of common stock in connection with private placement, net of placement agent fees and offering costs,shares		5,582,940
Issuance of common stock in connection with private placement, net of placement agent fees and offering costs	44,134	$ 6	44,128
Issuance of common stock in connection with a former employee letter agreement (Note 9), Shares		67,406
Issuance of common stock in connection with a former employee letter agreement (Note 9)	847		847
Forfeited restricted common stock	(1)		(1)
Forfeited restricted common stock, Shares		(1,334)
Exercise of stock options	519		519
Exercise of stock options, Shares		66,917
Unrealized gains on available-for-sale securities	219				219
Stock-based compensation expense	2,703		2,703
Net loss	$ (27,832)			(27,832)
Redeemable convertible preferred stock, shares outstanding, ending balance at Jun. 30, 2019	0
Ending balances at Jun. 30, 2019	$ 115,697	$ 51	$ 422,309	$ (307,233)	$ 570
Ending balance, shares at Jun. 30, 2019		51,010,368

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (27,832)	$ (27,503)	$ (61,030)	$ (48,712)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense			1,960	1,447
Stock-based compensation expense			5,509	4,258
Change in fair value of contingent consideration	4,889	7,852	16,591	12,760
Deferred income tax benefit			(486)	(2,602)
Accretion of discount on short-term investments				43
Changes in operating assets and liabilities, net of effects of acquisition:
Prepaid expenses and other assets			(1,766)	(2,509)
Right-of-use assets			234
Accounts payable			(1,858)	1,081
Accrued expenses			600	266
Deferred rent				387
Lease liability			(170)
Deferred revenue			(1,029)
Net cash used in operating activities			(41,445)	(33,581)
Cash flows from investing activities:
Purchases of investments			(38,438)	(6,000)
Sales and maturities of investments			55,756	14,918
Purchases of property and equipment			(1,793)	(4,757)
Net cash provided by investing activities			15,525	4,161
Cash flows from financing activities:
Proceeds from private placement, net of placement agent fees			44,608
Payments of private placement offering costs			(474)
Payments of initial public offering costs				(2,421)
Proceeds from option exercises			1,416	278
Net cash provided by (used in) financing activities			45,550	(2,143)
Net increase (decrease) in cash, cash equivalents and restricted cash			19,630	(31,563)
Cash, cash equivalents and restricted cash at beginning of period			56,224	50,024
Cash, cash equivalents and restricted cash at end of period	75,854	18,461	75,854	18,461
Cash, cash equivalents and restricted cash at end of period:
Cash and cash equivalents	74,904	16,495	74,904	16,495
Restricted cash	950	1,966	950	1,966
Total cash, cash equivalents and restricted cash at end of period	75,854	18,461	56,224	50,024
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses			59	489
Deferred offering costs included in accounts payable and accrued expenses			123	1,402
Issuance of common stock in connection with a former employee letter agreement (Note 9)	$ 847		$ 847
Accretion of redeemable convertible preferred stock to redemption value		$ 459		$ 644

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	6 Months Ended
	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Translate Bio, Inc. (the “Company”) is a clinical-stage messenger RNA (“mRNA”) therapeutics company developing a new class of potentially transformative medicines to treat diseases caused by protein or gene dysfunction. Using its proprietary mRNA therapeutic platform (“MRT platform”), the Company creates mRNA that encodes functional proteins. The Company’s mRNA is delivered to the target cell where the cell’s own machinery recognizes it and translates it, restoring or augmenting protein function to treat or prevent disease. The Company is initially focused on restoring the expression of intracellular and transmembrane proteins, areas that have eluded conventional protein therapeutics, in patients with genetic diseases where there is high unmet medical need. The Company is developing its lead MRT product candidate for the lung, MRT5005, for the treatment of cystic fibrosis (“CF”). The Company is conducting a Phase 1/2 clinical trial to evaluate the safety and tolerability of single and multiple-ascending doses of MRT5005. Percent predicted forced expiratory volume in one second (“ppFEV 1 1 The Company is developing its lead MRT product candidate for the liver, MRT5201, for the treatment of ornithine transcarbamylase (“OTC”) deficiency. In June 2019, the Company received clearance from the U.S. Food and Drug Administration (the “FDA”) to proceed with a SAD Phase 1/2 clinical trial of MRT5201 in adult patients with OTC deficiency. In December 2018, the Company had submitted an investigational new drug application (“IND”) for a SAD and MAD Phase 1/2 clinical trial of MRT5201, which the FDA placed on clinical hold, pending additional preclinical toxicology data. After discussions with the FDA and after the Company amended the protocol, the FDA removed the clinical hold and allowed the Company to move forward with a SAD clinical trial. The Company plans to initiate patient screening for the SAD Phase 1/2 clinical trial in the second half of 2019. The Company is currently conducting additional preclinical studies that are required to support future clinical development of MRT5201, including a MAD clinical trial, and plans to submit data from these preclinical studies to the FDA in the fourth quarter of 2019. The Company is subject to risks common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. The accompanying condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its two wholly owned subsidiaries, Translate Bio MA, Inc. and Translate Bio Securities Corporation, from their date of incorporation. All intercompany accounts and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated balance sheet as of June 30, 2019, the unaudited condensed consolidated statements of operations and of comprehensive loss for the three and six months ended June 30, 2019 and 2018, the unaudited condensed consolidated statements of redeemable convertible preferred stock and stockholders’ equity (deficit) for the three and six months ended June 30, 2019 and 2018 and of the unaudited condensed consolidated statements of cash flows for the six months ended June 30, 2019 and 2018 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. The accompanying balance sheet as of December 31, 2018 has been derived from the Company’s audited financial statements for the year ended December 31, 2018 previously filed with the SEC. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. The accompanying unaudited interim 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 that was filed with the SEC on March 21, 2019. The accompanying unaudited interim condensed consolidated financial presentation has been prepared on the same basis as the audited annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of June 30, 2019, the results of its operations for the three and six months ended June 30, 2019 and 2018, and its cash flows for the six months ended June 30, 2019 and 2018. The financial data and other information disclosed in these notes related to the three and six months ended June 30, 2019 and 2018 are also unaudited. The results for the three and six months ended June 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. Acquisition of Shire’s MRT Program In December 2016, the Company entered into an asset purchase agreement (as amended in June 2018, the “Shire Agreement”) with Shire Human Genetic Therapies, Inc. (“Shire”), a subsidiary of Takeda Pharmaceutical Company Ltd., pursuant to which Shire sold equipment to and assigned to the Company all of its rights to certain patent rights, permits, real property leases, contracts, regulatory documentation, books and records, and materials related to Shire’s mRNA therapy platform (the “MRT Program”), including its cystic fibrosis transmembrane conductance regulator (“CFTR”) and OTC deficiency mRNA therapeutic programs. Reverse Stock Split On June 15, 2018, the Company effected a one-for-5.5555 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s redeemable convertible preferred stock (see Note 8). Accordingly, all share and per share amounts for all periods presented in the accompanying condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and the associated adjustment of the preferred stock conversion ratios. Sales of Common Stock On July 2, 2018, the Company closed its initial public offering of its common stock (the “IPO”). In the IPO, the Company issued and sold 9,714,371 shares of common stock, including the underwriters’ over-allotment option, at a public offering price of $13.00 per share, resulting in aggregate net proceeds of $113.2 million after deducting underwriting discounts and commissions and offering expenses. On May 3, 2019, the Company issued and sold 5,582,940 shares of its common stock in a private placement at a price per share of $8.50, resulting in gross proceeds of $47.5 million, before deducting placement agent fees of $2.8 million and other offering expenses of $0.6 million. Sanofi Pasteur Collaboration and Licensing Agreement In 2018, the Company entered into a collaboration and license agreement with Sanofi Pasteur Inc. (“Sanofi”), the vaccines global business unit of Sanofi S.A., to develop mRNA vaccines for up to five infectious disease pathogens (the “Sanofi Agreement”). Under the Sanofi Agreement, the Company and Sanofi are jointly conducting research and development activities to advance mRNA vaccines and mRNA vaccine platform development during a three-year research term, which may be extended by mutual agreement. Following the research term, the Company is obligated to manufacture clinical product for Sanofi, which the Company estimates may take up to eight years to complete. The Company is eligible to receive up to $805.0 million in payments, including an upfront payment of $45.0 million, which the Company received in 2018; certain development, regulatory and sales-related milestones across several vaccine targets; and option exercise fees if Sanofi exercises its option related to development of vaccines for additional pathogens. The Company is also eligible to receive reimbursable development costs and tiered royalty payments associated with worldwide sales of the developed vaccines, if any (see Note 3). Going Concern In accordance with Accounting Standards Update (“ASU”) No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40) The Company’s financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. Through June 30, 2019, the Company has funded its operations with proceeds from the sale of redeemable convertible preferred stock and the sale of bridge units, which ultimately converted into shares of the Company’s common stock, the proceeds from the IPO, the upfront payment received under the Sanofi Agreement and the proceeds from a private placement of the Company’s common stock. The Company has incurred recurring losses and cash outflows from operations since its inception, including net losses of $61.0 million and $48.7 million for the six months ended June 30, 2019 and 2018, respectively. In addition, the Company had an accumulated deficit of $307.2 million as of June 30, 2019. The Company expects to continue to generate operating losses for the foreseeable future. As of July 31, 2019, the date of issuance of these unaudited interim condensed consolidated financial statements, the Company expects that its cash, cash equivalents and short-term investments of $146.9 million as of June 30, 2019 will be sufficient to fund its operating expenses and capital expenditure requirements into the second half of 2020. The future viability of the Company beyond that point is dependent on the Company’s ability to raise additional capital to finance its operations. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all. The Company expects that its expenses will increase in connection with its ongoing business activities. As a result, the Company will need substantial additional funding to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through the sale of equity, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may be unable to raise additional funds or enter into such other agreements or arrangements when needed on favorable terms, or at all. If the Company is unable to obtain funding, the Company will be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. Based on its recurring losses and cash outflows from operations since inception, expectation of continuing operating losses and cash outflows from operations for the foreseeable future and the need to raise additional capital to finance its future operations, the Company concluded that there was substantial doubt about its ability to continue as a going concern.

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 The significant accounting policies and estimates used in preparation of the consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. During the six months ended June 30, 2019, there were no material changes to the Company’s significant accounting policies, except for the adoption of ASU No. 2016-02, Leases (Topic 842) Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (the “FASB”) issued ASU 2016-02, which requires lessees to recognize most leases on their balance sheet as a right-of-use (“ROU”) asset and a lease liability. The Company adopted ASU 2016-02 as of the required effective date of January 1, 2019 using the cumulative-effect adjustment on the effective date of the standard, with comparative periods presented in accordance with the previous guidance in ASC 840. Subsequent to the issuance of Topic 842, the FASB clarified the guidance through several ASUs; hereinafter the collection of lease guidance is referred to as “ASC 842”. The Company elected the permitted practical expedients within ASC 842, which allowed the Company to not reassess previous accounting conclusions around whether arrangements are or contain leases, and carried forward both the historical classification of leases and the treatment of initial direct costs. In addition, the Company elected to exclude leases with an initial term of one year or less in the recognized ROU assets and lease liabilities. Adoption of the new standard resulted in the recording of ROU assets and related lease liabilities of approximately $10.9 million and $13.0 million, respectively, as of January 1, 2019. The standard did not materially impact the Company’s consolidated net earnings and had no impact on cash flows. Refer to Note 12 for the additional disclosures required by ASC 842. The Company determines if an arrangement is a lease at inception. For leases where the Company is the lessee, ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent an obligation to make lease payments arising from the lease. ROU assets and lease liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. As most of the Company’s leases do not provide an implicit interest rate, the Company uses its incremental borrowing rate, which are the rates incurred to borrow on a collateralized basis over a term equal to the lease payments in a similar economic environment, in determining the present value of lease payments. The lease terms used to calculate the ROU asset and related lease liability include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for operating leases is recognized on a straight-line basis over the lease term as an operating expense. The Company has lease agreements which require payments for lease and non-lease components and has elected to account for these as a single lease component. In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception ( ) . In June 2018, the FASB issued ASU No. 2018-07, Compensation-Stock Compensation (Topic 718) Recently Issued Accounting Pronouncements In June 2016, the FASB issued guidance on the Measurement of Credit Losses on Financial Instruments. The guidance requires that credit losses be reported using an expected losses model rather than the incurred losses model that is currently used, and establishes additional disclosures related to credit risks. For available-for-sale debt securities with unrealized losses, the standard now requires allowances to be recorded instead of reducing the amortized cost of the investment. This standard will be effective for the Company on January 1, 2020. The Company does not expect that the adoption of this new standard will have a material impact on its disclosures. In January 2017, the FASB issued ASU No. 2017-04, Intangibles—Goodwill and Other: Simplifying the Test for Goodwill Impairment (Topic 350) In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808) Revenue Recognition Collaborative Arrangements

Sanofi Collaboration and Licens

Sanofi Collaboration and License Agreement	6 Months Ended
	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Sanofi Collaboration and License Agreement	3. Sanofi Collaboration and License Agreement In 2018, the Company entered into the Sanofi Agreement, a collaboration and license agreement with Sanofi to develop mRNA vaccines and mRNA vaccine platform development for up to five infectious disease pathogens (the “Licensed Fields”). Under the Sanofi Agreement, the Company and Sanofi have agreed to collaborate to perform certain research and development activities to advance mRNA vaccines and mRNA vaccine platform development during a three-year research term, which may be extended by mutual agreement. Following the research term, the Company is obligated to manufacture clinical product for Sanofi, which the Company estimates may take up to eight years to complete. The collaboration activities will be subject to a collaboration plan to be updated annually. Under the terms of the Sanofi Agreement, the Company received an upfront payment of $45.0 million and is eligible for certain potential milestone and option payments, each as further described below. In addition, the Company is eligible to receive from Sanofi tiered royalty payments on worldwide net sales of mRNA vaccines. Under the Sanofi Agreement, the Company and Sanofi created a governance structure, including committees and working groups, to manage the activities under the collaboration. If the Company and Sanofi do not mutually agree on certain decisions, Sanofi would be able to break a deadlock without the Company’s consent. The collaboration includes an estimated budget. Sanofi is responsible for paying reimbursable development costs, including the Company’s employee costs, out-of-pocket costs paid to third parties and manufacturing costs, up to a specified amount. Any reimbursable development costs are payable by Sanofi within 60 days of invoicing. Under the terms of the Sanofi Agreement, the Company has granted to Sanofi exclusive, worldwide licenses under applicable patents, patent applications, know-how and materials, including those arising under the collaboration, to develop, commercialize and manufacture mRNA vaccines to prevent, treat or cure diseases, disorders or conditions in humans caused by any of three of the Licensed Fields. In addition, pursuant to the terms of the Sanofi Agreement and subject to certain limitations, Sanofi has options to add up to two additional infectious disease pathogens within the granted licenses to the Licensed Fields by exercising either option or both options during a specified option term and paying the Company a $5.0 million fee per added pathogen. If, prior to the exercise of the options by Sanofi, the Company receives a bona fide third-party offer to acquire rights to the field to which an option relates, the Company must notify Sanofi of such offer, and if Sanofi does not exercise its option as to the applicable field, such field will no longer be subject to the option. The Company and Sanofi retain the rights to perform their respective obligations and exercise their respective rights under the Sanofi Agreement, and Sanofi may grant sublicenses to affiliates or third parties. Sanofi has also granted the Company non-exclusive, sublicensable licenses under patent rights claiming certain improvements that Sanofi may make to the technology the Company has licensed to it or claiming certain technology arising from the collaboration and owned by Sanofi. The Company may exercise such licenses to develop, manufacture and commercialize products, other than products that use a vaccine to prevent, treat or cure a disease, disorder or condition in humans caused by an infectious disease pathogen. If the Company commercializes any product covered by such a Sanofi patent right, the Company would pay Sanofi a royalty of a low single-digit percentage. Sanofi may terminate these licenses to the Company if the Company materially breaches the terms of the license and the breach remains uncured for a specified period, which may be extended in certain circumstances. Sanofi has sole responsibility for all commercialization activities for mRNA vaccines in the Licensed Fields and is obligated to bear all costs in connection with any such commercialization. The Company and Sanofi intend to enter into a supply agreement pursuant to which the Company would be responsible for manufacturing certainnon-clinical and clinical mRNA vaccines and materials containing mRNA until the Company transfers such manufacturing capabilities to Sanofi. The Company would be entitled to receive payments for manufacturing mRNA vaccines under the supply agreement. The Sanofi Agreement provides that the Company is eligible to receive aggregate potential payments of up to $805.0 million from Sanofi, which includes an upfront payment, potential milestone payments and potential option exercise payments. In 2018, Sanofi paid the Company a $45.0 million upfront payment in respect of the licenses and options granted to Sanofi. Sanofi will also pay the Company $5.0 million with respect to each additional Licensed Field for which it exercises an option. Sanofi has also agreed to pay the Company milestone payments upon the achievement of specified development, regulatory and commercialization milestones. In particular, the Company is entitled to receive development and regulatory milestone payments of up to $63.0 million per Licensed Field and sales milestone payments of up to $85.0 million per Licensed Field. In addition, the Company is entitled to receive a $10.0 million milestone payment from Sanofi following completion of the technology and process transfer. Sanofi has agreed to pay the Company a tiered royalty on worldwide net sales of all mRNA vaccines within each Licensed Field ranging from a high single-digit percentage to a low teens percentage, depending on quarterly net sales by Sanofi, its affiliates and its sublicensees. The royalty paid to the Company can be reduced with respect to a product once the relevant licensed patent rights expire or if additional licensed technology is required, but the royalty payments generally may not fall below the Company’s royalty obligations to third parties plus a royalty of a low single-digit percentage. Royalty payments under the Sanofi Agreement are payable on a product-by-product and country-by-country basis beginning on the launch of the product in the country until the later of the expiration of the last valid claim covering such product or 10 years after the launch of such product in such country. The Sanofi Agreement provides that it will remain in effect until terminated in accordance with its terms. Either the Company or Sanofi may terminate the Sanofi Agreement in its entirety if the other party is subject to certain insolvency proceedings. Either party may terminate the Sanofi Agreement in its entirety or with respect to a particular Licensed Field, country or product if the other party materially breaches the Sanofi Agreement and the breach remains uncured for a specified period, which may be extended in certain circumstances. Sanofi may also terminate the Sanofi Agreement in its entirety or with respect to a particular Licensed Field, country or product for safety reasons or for convenience, in each case after a specified notice period. After termination of the Sanofi Agreement, Sanofi may continue to manufacture and commercialize the terminated products for a specified period of time, subject to Sanofi’s payment obligations. Accounting Under ASC 606 In determining the appropriate amount of revenue to be recognized under Accounting Standard Codification (“ASC”) 606, Revenue from Contracts with Customers The Company identified the following promised goods or services contained in the Sanofi Agreement: (i) the license it conveyed to Sanofi with respect to the Licensed Fields, (ii) the licensed know-how to be conveyed to Sanofi with respect to the Licensed Fields, (iii) its obligation to perform research and development on the Licensed Fields, (iv) its obligation to transfer licensed materials to Sanofi, (v) its obligation to manufacture and supply certain non-clinical and clinical mRNA vaccines and materials containing mRNA until the Company transfers such manufacturing capabilities to Sanofi and (vi) the technology and process transfer. The Company assessed whether each of these promised goods or services are distinct performance obligations on their own or if they need to be combined with other promises to create a bundle that is a distinct performance obligation. The Company determined that the promised goods and services do not have standalone value and are highly interrelated. Accordingly, the promised goods and services represent one performance obligation. Sanofi’s right to exercise options for up to two additional infectious disease pathogens within the granted licenses to the Licensed Fields are accounted for separately as they do not represent material rights, based on the criteria of ASC 606. Upon the exercise of any option by Sanofi, the contract promises associated with an option target would use a separate proportional performance model for purposes of revenue recognition under ASC 606. There was no significant financing component or non-cash consideration included in the Sanofi Agreement. Under ASC 606, at the end of each reporting period, the Company re-evaluates the probability that the consideration associated with each milestone or reimbursement will not be subject to a significant reversal in the cumulative amount of revenue recognized, and, if necessary, adjusts the estimate of the overall transaction price. During the three months ended March 31, 2019, the Company reduced the overall transaction price by $10.0 million. The transaction price includes the upfront, non-refundable payment of $45.0 million for the transfer of the combined license, supply and development obligations under the Sanofi Agreement, an estimated $32.6 million in reimbursable employee costs, an estimated $54.5 million in reimbursable development costs including out-of-pocket costs paid to third parties and manufacturing costs and an estimated $19.0 million in milestone payments. Under ASC 606, the Company recognized revenue using the cost-to-cost input method, which it believes best depicts the transfer of control to the customer. Under the cost-to-cost input 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 as a percentage of the estimated transaction price based on the extent of progress towards completion. The estimate of the Company’s measure of progress and estimate of variable consideration to be included in the transaction price will be updated at each reporting date as a change in estimate. The amount related to the unsatisfied portion will be recognized as that portion is satisfied over time. The following table summarizes the Company’s collaboration revenue (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Collaboration revenue $ 1,174 $ — $ 2,648 $ — The following table presents the balance of the Company’s contract liabilities (in thousands): June 30, December 31, Contract liabilities Deferred revenue $ 43,207 $ 44,413 The Company considers the total consideration expected to be earned in the next 12 months for services to be performed as short-term deferred revenue, and consideration that is expected to be earned subsequent to 12 months from the balance sheet date as long-term deferred revenue. The Company expects to complete its obligations and recognize all net revenues from the collaboration over eight years. Revenue recognized from contract liabilities was $1.2 million during the six months ended June 30, 2019. Included in prepaid expenses and other current assets on the condensed consolidated balance sheets as of June 30, 2019 and December 31, 2018 were $0.7 million and $0.8 million, respectively, of short-term receivables from Sanofi for reimbursable development costs.

Intangible Assets and Goodwill

Intangible Assets and Goodwill	6 Months Ended
	Jun. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	4. Intangible Assets and Goodwill Intangible Assets, Net The acquisition of Shire’s MRT Program was accounted for in accordance with the acquisition method of accounting for business combinations. The total purchase consideration transferred was allocated to the tangible and identifiable intangible assets acquired based on their estimated fair values. The tables below present the Company’s definite-lived intangible assets that are subject to amortization and indefinite-lived intangible assets: June 30, 2019 Estimated Gross Carrying Amount Accumulated Amortization Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 years $ 45,992 $ (1,212 ) $ 44,780 Indefinite-lived intangible assets: IPR&D—CF Indefinite 42,291 — 42,291 IPR&D—OTC Indefinite 18,559 — 18,559 Total intangible assets, net $ 106,842 $ (1,212 ) $ 105,630 December 31, 2018 Estimated Gross Carrying Amount Accumulated Amortization Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 years $ 45,992 $ (397 ) $ 45,595 Indefinite-lived intangible assets: IPR&D—CF Indefinite 42,291 — 42,291 IPR&D—OTC Indefinite 18,559 — 18,559 Total intangible assets, net $ 106,842 $ (397 ) $ 106,445 Identifiable intangible assets acquired in the acquisition of Shire’s MRT Program consisted of in-process research and development (“IPR&D”), which included ongoing projects that could further the Company’s preclinical and clinical development activities related to CF, OTC deficiency and other potential rare diseases. As of the date of acquisition, the IPR&D was determined to be indefinite-lived. Upon commencement of the Sanofi Agreement, the IPR&D—MRT intangible asset was reclassified from indefinite-lived to definite-lived intangible assets and the Company began amortization of this intangible asset. Amortization will be recorded over an estimated eight-year period based on an economic consumption model. For the three and six months ended June 30, 2019, the Company recorded amortization expense of $0.3 million and $0.8 million, respectively, related to the definite-lived MRT intangible asset. The estimated aggregate amortization expense for each of the five succeeding fiscal years is $3.6 million, $9.0 million, $9.9 million, $4.2 million, and $2.3 million for the years ending December 31, 2019, 2020, 2021, 2022 and 2023, respectively. Indefinite-lived IPR&D is not subject to amortization, but is tested annually for impairment or more frequently if there are indicators of impairment. The Company tests its indefinite-lived IPR&D annually for impairment on October 1st. In December 2018, the Company submitted an IND to the FDA to support the initiation of a Phase 1/2 clinical trial for MRT5201 in patients with OTC deficiency. In January 2019, the FDA notified the Company that its IND for MRT5201 was placed on clinical hold. The Company determined this clinical hold was an indicator of impairment and as a result, retested the indefinite-lived IPR&D related to the OTC deficiency program for impairment. The Company performed a quantitative impairment analysis whereby the Company forecasted the net cash flows expected to be generated by the indefinite-lived IPR&D related to the OTC deficiency program over its estimated useful life. The net cash flows reflected the program’s stage of completion, the probability of technical success, the projected costs to complete, the expected market competition and an assessment of the program’s life-cycle. The net cash flows were then adjusted to present value by applying an appropriate discount rate that reflects the risk factors associated with the cash flow streams. Following this retest the Company determined the indefinite-lived IPR&D related to the OTC deficiency program was not impaired. Therefore, the Company did not recognize an impairment charge. There were no indicators of impairment during the three months ended June 30, 2019. Goodwill The excess of the fair value of the consideration transferred over the fair value of identifiable assets acquired in the acquisition of Shire’s MRT Program was allocated to goodwill in the amount of $21.4 million. There have been no changes to the carrying amount of goodwill during the six months ended June 30, 2019. Goodwill is not subject to amortization, but is tested annually for impairment or more frequently if there are indicators of impairment. The Company tests its goodwill annually for impairment on October 1 st

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	5. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis (in thousands): Fair Value Measurements as of June 30, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 17,554 $ — $ 17,554 U.S. government agency bonds — 71,961 — 71,961 $ — $ 89,515 $ — $ 89,515 Liabilities: Contingent consideration $ — $ — $ 120,233 $ 120,233 $ — $ — $ 120,233 $ 120,233 Fair Value Measurements as of December 31, 2018 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 23,318 $ — $ 23,318 U.S. government agency bonds — 88,904 — 88,904 $ — $ 112,222 $ — $ 112,222 Liabilities: Contingent consideration $ — $ — $ 103,642 $ 103,642 $ — $ — $ 103,642 $ 103,642 During the six months ended June 30, 2019 and the year ended December 31, 2018, there were no transfers between Level 1, Level 2 and Level 3. Cash equivalents as of June 30, 2019 and December 31, 2018 consisted of money market funds totaling $17.6 million and $23.3 million, respectively. The money market funds were valued using inputs observable in active markets for similar securities, which represent a Level 2 measurement in the fair value hierarchy. The Company’s short-term investments as of June 30, 2019 and December 31, 2018 consisted of U.S. government agency bonds and were classified as available-for-sale securities. The U.S. government agency bonds were valued using inputs observable in active markets for similar securities, which represent a Level 2 measurement in the fair value hierarchy. As of June 30, 2019, the Company’s short-term investments had an amortized cost of $71.4 million, an unrealized gain of $0.6 million and a fair value of $72.0 million. All of these securities have a maturity of one year or less. Valuation of Contingent Consideration The contingent consideration liability related to the acquisition of Shire’s MRT Program in 2016 was classified as Level 3 measurement within the fair value hierarchy. The Company may be required to pay future consideration contingent upon the achievement of potential future milestones and earnout payments that may be due by the Company to Shire. The fair value of the liability to make potential future milestone and earnout payments was estimated by the Company at each reporting date based, in part, on the results of a third-party valuation using a discounted cash flow analysis based on various assumptions, including the probability of achieving specified events, discount rates, and the period of time until earnout payments are payable and the conditions triggering the milestone payments are met. The actual settlement of contingent consideration could differ from current estimates based on the actual occurrence of these specified events. The following table presents the unobservable inputs and fair value of the components of the contingent consideration (dollar amounts in thousands): Unobservable Inputs at June 30, 2019 and December 31, 2018 Fair Value at Projected Year of Payment June 30, December 31, Earnout payments 2025 - 2039 $ 110,770 $ 94,999 Milestone payments 2025 - 2030 9,463 8,643 $ 120,233 $ 103,642 The discount rate used in the third-party valuation was 13.5% and 14.5% as of June 30, 2019 and December 31, 2018, respectively. The following table presents a roll-forward of the total acquisition-related contingent consideration liability (in thousands): Fair Value Balance as of December 31, 2018 $ 103,642 Change in fair value of contingent consideration 16,591 Balance as of June 30, 2019 $ 120,233 The increase in the fair value of contingent consideration during the six months ended June 30, 2019 was primarily due to the continued progress of MRT5005 and MRT5201, the time value of money due to the passage of time and a decrease in the discount rate.

Property and Equipment, Net

Property and Equipment, Net	6 Months Ended
	Jun. 30, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	6. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): June 30, December 31, Laboratory equipment $ 8,394 $ 7,012 Computer equipment 780 686 Office equipment 883 836 Leasehold improvements 5,634 5,635 Construction in progress 1,241 959 16,932 15,128 Less: Accumulated depreciation and amortization (6,029 ) (4,883 ) $ 10,903 $ 10,245 Depreciation and amortization expense related to property and equipment was $0.6 million, $0.9 million, $1.1 million and $1.4 million for the three months ended June 30, 2019 and 2018 and the six months ended June 30, 2019 and 2018, respectively.

Accrued Expenses

Accrued Expenses	6 Months Ended
	Jun. 30, 2019
Payables and Accruals [Abstract]
Accrued Expenses	7. Accrued Expenses Accrued expenses consisted of the following (in thousands): June 30, December 31, Accrued external research and development expenses $ 2,950 $ 1,901 Accrued employee compensation and benefits 2,392 2,933 Accrued consultant and professional fees 1,904 977 Other — 736 $ 7,246 $ 6,547

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	6 Months Ended
	Jun. 30, 2019
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	8. Redeemable Convertible Preferred Stock As of December 31, 2017, the Company had 142,288,292 shares of redeemable convertible preferred stock issued and outstanding which were redeemable and convertible by the holders under specified conditions. The redeemable convertible preferred stock was classified outside of stockholders’ equity (deficit) because the shares contained redemption features that were not solely within the control of the Company. Upon the closing of the Company’s IPO on July 2, 2018, all then-outstanding shares of redeemable convertible preferred stock converted into an aggregate of 25,612,109 shares of common stock according to their terms. As of June 30, 2019 and December 31, 2018, there were no shares of redeemable convertible preferred stock authorized, issued or outstanding.

Incentive Stock Options and Res

Incentive Stock Options and Restricted Stock	6 Months Ended
	Jun. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Incentive Stock Options and Restricted Stock	9. Incentive Stock Options and Restricted Stock 2018 Equity Incentive Plan On March 7, 2018, the Company’s board of directors, subject to stockholder approval, adopted, and on June 15, 2018, its stockholders approved, the 2018 Equity Incentive Plan (the “2018 Plan”), which became effective on June 27, 2018. The 2018 Plan provides for the grant of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2018 Plan is the sum of 2,512,187, plus the number of shares (up to 1,013,167 shares) equal to the sum of (i) the number of shares remaining available for issuance under the 2016 Stock Incentive Plan, as amended, (the “2016 Plan”), upon the effectiveness of the 2018 Plan, which was 360,514 shares, and (ii) the number of shares of common stock subject to outstanding awards under the 2016 Plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right. The number of shares of common stock that may be issued under the 2018 Plan will automatically increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2019 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2028, by an amount equal to the lowest of (i) 3,349,582 shares, (ii) 4% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors. Accordingly, on January 1, 2019, the number of shares of common stock that may be issued under the 2018 Plan increased by 1,805,598 shares of common stock and through June 30, 2019, a total of 87,364 shares issued under the 2016 plan have been cancelled for a total of 4,765,663 shares of common stock reserved for issuance under this plan as of June 30, 2019. The shares of common stock underlying any awards that are forfeited, canceled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, repurchased or are otherwise terminated by the Company under the 2018 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan. The 2018 Plan is administered by the board of directors. The exercise prices, vesting periods and other restrictions are determined at the discretion of the board of directors, except that the exercise price per share of options may not be less than 100% of the fair market value of the common stock on the date of grant. Stock options awarded under the 2018 Plan expire 10 years after the grant date, unless the board of directors sets a shorter term. Awards granted to employees, officers, members of the board of directors and consultants typically vest over a period of one to four years. Typically, unvested stock options are forfeited upon the recipient ceasing to provide services to the Company. 2018 Employee Stock Purchase Plan On March 7, 2018, the Company’s board of directors, subject to stockholder approval, adopted, and on June 15, 2018, its stockholders approved the 2018 Employee Stock Purchase Plan (the “2018 ESPP”), which became effective on June 27, 2018. A total of 418,697 shares of common stock were initially reserved for issuance under this plan. The number of shares of common stock that may be issued under the 2018 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year commencing on January 1, 2019 and continuing for each fiscal year until, and including, the fiscal year commencing on January 1, 2029, by an amount equal to the lowest of (i) 837,395 shares, (ii) 1% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors. Accordingly, on January 1, 2019, the number of shares of common stock that may be issued under the 2018 ESPP increased by 451,399 shares for a total of 870,096 shares of common stock reserved for issuance under this plan. As of June 30, 2019, no shares had been issued under the 2018 ESPP. 2016 Stock Incentive Plan The 2016 Plan provides for the grant of stock options, stock appreciation rights, restricted stock and restricted stock units. Shares that are expired, terminated, surrendered or canceled under the 2016 Plan without having been exercised will be available for future grants of awards under the 2018 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards under the 2018 Plan. The 2016 Plan is administered by the board of directors. The exercise prices, vesting periods and other restrictions were determined at the discretion of the board of directors, except that the exercise price per share of options could not be less than 100% of the fair market value of the common stock on the date of grant. Stock options awarded under the 2016 Plan expire 10 years after the grant date, unless the board of directors set a shorter term. Stock options and restricted stock granted to employees, officers, members of the board of directors and consultants typically vest over a four-year period. Upon the effectiveness of the 2018 Plan on June 27, 2018, no further awards will be made under the 2016 Plan, but awards outstanding under the 2016 Plan will continue to be governed by their existing terms. Stock Options The following table summarizes the Company’s stock option activity since December 31, 2018 (in thousands, except share and per share amounts): Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Intrinsic Value (in years) Outstanding as of December 31, 2018 6,236,006 $ 7.78 8.74 $ 1,104 Granted 2,614,650 $ 8.48 Exercised (221,401 ) $ 6.40 Forfeited (80,595 ) $ 7.85 Outstanding as of June 30, 2019 8,548,660 $ 8.03 8.83 $ 39,344 Exercisable as of June 30, 2019 2,671,285 $ 7.59 8.43 $ 13,472 Vested and expected to vest as of June 30, 2019 8,548,660 $ 8.03 8.83 $ 39,344 Exercisable as of December 31, 2018 1,827,004 $ 7.16 7.95 $ 632 Vested and expected to vest as of December 31, 2018 6,236,006 $ 7.78 8.74 $ 1,104 The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the six months ended June 30, 2019 and 2018 was $0.6 million and $0.3 million, respectively. The weighted average grant-date fair value per share of stock options granted was $5.56 and $5.91 during the six months ended June 30, 2019 and 2018, respectively. Stock Option Valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees and directors: Six Months Ended June 30, 2019 2018 Risk-free interest rate 2.42 % 2.80 % Expected term (in years) 6.0 6.0 Expected volatility 73.3 % 75.7 % Expected dividend yield 0 % 0 % Restricted Common Stock The following table summarizes the Company’s restricted stock activity since December 31, 2018: Number of Shares Weighted Average Grant-Date Fair Value Unvested restricted common stock outstanding as of December 31, 2018 219,148 $ 1.27 Forfeited restricted common stock (1,334 ) $ 1.28 Vested restricted common stock (97,826 ) $ 1.27 Unvested restricted common stock outstanding as of June 30, 2019 119,988 $ 1.28 Stock-Based Compensation Stock-based compensation expense was classified in the condensed consolidated statements of operations as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Research and development expenses $ 1,284 $ 1,671 $ 2,153 $ 2,453 General and administrative expenses 2,265 1,204 3,356 1,805 $ 3,549 $ 2,875 $ 5,509 $ 4,258 Included in general and administrative stock-based compensation expense during each of the three and six months ended June 30, 2019 is $0.8 million related to the issuance of 67,406 shares of common stock in connection with a former employee letter agreement. As of June 30, 2019, total unrecognized compensation cost related to the unvested stock-based awards was $29.3 million, which is expected to be recognized over a weighted average period of 2.75 years.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes The Company recognized an income tax benefit of $1.5 million during the three months ended June 30, 2018 and $0.5 million and $2.6 million during the six months ended June 30, 2019 and 2018, respectively. There was no income tax benefit recognized during the three months ended June 30, 2019. The income tax benefits recognized during the three months ended June 30, 2018 and six months ended June 30, 2019 and 2018 resulted from a reduction in the deferred tax liabilities recorded as part of the Company’s acquisition of the MRT Program as well as deferred tax assets recorded for net operating losses generated that have an unlimited carryforward period. Net operating losses generated in 2018 and years thereafter can be carried forward indefinitely. The reduction in the deferred tax liabilities during the three months ended June 30, 2018 and six months ended June 30, 2019 and 2018 resulted from an increase in the tax basis of the indefinite-lived IPR&D recorded in the acquisition.

Net Loss per Share

Net Loss per Share	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Net Loss per Share	11. Net Loss per Share Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Numerator: Net loss $ (27,832 ) $ (27,503 ) $ (61,030 ) $ (48,712 ) Accretion of redeemable convertible preferred stock to redemption value — (459 ) — (644 ) Net loss attributable to common stockholders $ (27,832 ) $ (27,962 ) $ (61,030 ) $ (49,356 ) Denominator: Weighted average common shares outstanding—basic and diluted 48,749,627 9,187,207 46,866,842 9,139,638 Net loss per share attributable to common stockholders—basic and diluted $ (0.57 ) $ (3.04 ) $ (1.30 ) $ (5.40 ) Due to a clerical error in the computation of the net loss attributable to common stockholders, the Company reported the net loss per share applicable to common stockholders—basic and diluted for the three and six months ended June 30, 2018 as $2.94 and $5.26, respectively, in the Company’s Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2018. The net loss per share applicable to common stockholders—basic and diluted for the three and six months ended June 30, 2018 was $3.04 and $5.40, respectively, as shown in the chart above. The Company concluded that this error was not material to the periods impacted. The Company excluded 147,914 shares and 412,040 shares of restricted common stock, presented on a weighted average basis, from the calculations of basic net loss per share attributable to common stockholders for the three months ended June 30, 2019 and 2018, respectively, because those shares had not vested. The Company excluded 172,941 shares and 451,286 shares of restricted common stock, presented on a weighted average basis, from the calculations of basic net loss per share attributable to common stockholders for the six months ended June 30, 2019 and 2018, respectively, because those shares had not vested. The Company’s potentially dilutive securities, which include stock options, unvested restricted common stock and redeemable convertible preferred stock, have been excluded from the computation of diluted net loss per share attributable to common stockholders as the effect would be to reduce the net loss per share. Therefore, the weighted average number of shares of common stock 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 shares of common stock, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: June 30, 2019 2018 Options to purchase common stock 8,548,660 6,093,408 Unvested restricted common stock 119,988 368,875 Redeemable convertible preferred stock (as converted to common stock) — 25,612,109 8,668,648 32,074,392 In addition to the potentially dilutive securities noted above, as of June 30, 2018, the Company was obligated to issue common stock to Shire upon the occurrence of specified events. Because the necessary conditions for issuance of the shares had not been met as of June 30, 2018, the Company excluded these shares from the table above and from the calculations of diluted net loss per share for the three and six months ended June 30, 2018. The Company’s obligation to issue additional shares of its common stock to Shire was satisfied upon completion of the Company’s IPO.

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Leases	12. Leases The Company is a lessee under two operating leases comprising a commercial real estate lease and an equipment lease. Real Estate Lease In June 2017, the Company entered into an operating lease for office and laboratory space at its headquarters in Lexington, Massachusetts. The Company occupies approximately 59,000 square feet of space under a 10-year lease agreement expiring in April 2028. The Company occupied this property in March 2018. Monthly lease payments include base rent charges of $0.2 million, which are subject to a 3% annual increase each year. In June 2017, in connection with this lease agreement, the Company issued a letter of credit collateralized by cash deposits of $1.0 million, which are classified as restricted cash on the consolidated balance sheets as of June 30, 2019 and December 31, 2018. Equipment Lease In March 2018, the Company entered into an operating lease for communications equipment for use at its office and laboratory space in Lexington, Massachusetts. The term of the lease is five years, expiring in March 2023. The Company excludes leases with an initial term of one year or less in the recognized ROU assets and lease liabilities. The Company recognizes lease expense for these leases on a straight-line basis over the lease term. For lease agreements entered into or reassessed after the adoption of ASC 842, lease and non-lease components are combined into a single lease component. The Company’s leases have remaining lease terms of up to nine years, excluding two five-year options to extend the real estate lease after the expiration of the initial term. The Company believes this office and laboratory space will be sufficient to meet its needs for the foreseeable future and that suitable additional space will be available as and when needed. The components of lease cost were as follows (dollar amounts in thousands): Three Months Ended Six Months Ended Lease Cost Operating lease cost $ 673 $ 1,346 Short-term lease cost — — Total lease cost $ 673 $ 1,346 Other Information Operating cash flows from operating leases $ 650 $ 1,282 Operating lease liabilities arising from obtaining right-of-use assets $ — $ — Weighted-average remaining lease term 9 years 9 years Weighted-average discount rate 17.5 % 17.5 % During the three and six months ended June 30, 2018, the company recorded rent expense of $0.8 million and $1.7 million, respectively. As of June 30, 2019, minimum rental commitments under these leases are as follows (in thousands): June 30, 2019 2019 $ 1,301 2020 2,659 2021 2,737 2022 2,818 2023 2,860 2024 and thereafter 13,096 Total future minimum lease payments 25,471 Less: imputed interest (12,652 ) Present value of lease liabilities $ 12,819 As of December 31, 2018, minimum rental commitments under the real estate lease was as follows (in thousands): December 31, 2018 2019 $ 2,534 2020 2,610 2021 2,688 2022 2,769 2023 2,852 2024 and thereafter 13,096 Total future minimum lease payments $ 26,549 As most of the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate which are the rates incurred to borrow on a collateralized basis over a term equal to the lease payments in a similar economic environment in determining the present value of lease payments. The Company used the incremental borrowing rate on January 1, 2019 for operating leases that commenced prior to that date.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies Research, Supply and License Agreements Pursuant to the Shire Agreement in December 2016, the Company was assigned and assumed several contracts related to the MRT Program. The material agreements that were assigned to and assumed by the Company in connection with the acquisition are described below. Roche Master Supply Agreement The Company is a party to a master supply agreement with Roche Diagnostics Corporation (“Roche”) pursuant to which Roche will custom manufacture certain products for the Company. The agreement requires the Company to purchase from Roche specified manufactured products and the related raw materials in an amount equal to the greater of (i) quantities of raw materials in the Company’s annual forecast to be purchased or (ii) 80% of the Company’s demand for products as the same or similar type (the “Purchase Commitment”). In June 2017, the Company exercised its option under the agreement to extend the agreement through December 31, 2024. In September 2018, the Company and Roche amended the agreement to remove and replace the Purchase Commitment for certain manufactured products and related raw materials supplied by Roche. The agreement, as amended, specifies a minimum purchase requirement for certain custom manufactured products. As of June 30, 2019, the Company’s purchase commitments under the agreement totaled $20.4 million, with $6.0 million committed as payments for the remainder of 2019, $0.5 million committed as payments in 2020 and $3.5 million committed as payments each year from 2021 to 2024. Research and development expenses related to this agreement totaled $1.0 million and $0.8 million during the three months ended June 30, 2019 and 2018, respectively, and $3.5 million and $1.7 million during the six months ended June 30, 2019 and 2018, respectively. MIT Research Agreement The Company is a party to a research agreement with the Massachusetts Institute of Technology (“MIT”) pursuant to which the Company is obligated to reimburse MIT in an amount up to $3.1 million for specified direct and indirect costs incurred through October 2019 in specified research activities conducted for the Company. As of June 30, 2019 and December 31, 2018, the Company had paid MIT $3.1 million and $2.5 million, respectively, of the total committed amount. Research and development expenses related to this agreement totaled $0.2 million and $0.3 million during the three months ended June 30, 2019 and 2018, respectively, and $0.5 million and $0.6 million during the six months ended June 30, 2019 and 2018, respectively. As of June 30, 2019 and December 31, 2018, there were no amounts payable by the Company under the agreement. As amended, the agreement expires in October 2019 and may be extended thereafter by mutual agreement of the parties. MIT Exclusive Patent License Agreement The Company is a party to an exclusive patent license agreement with MIT pursuant to which the Company received an exclusive license under the licensed patent rights to develop, manufacture and commercialize any product containing both (i) any RNA sequences, including mRNA, that encode a protein or peptide suitable for human therapeutic use which may include operably linked non-coding sequences that facilitate translation of the coding portion of such RNA sequence, but such non-coding sequences do not include nucleic acids that function through an RNA interface mechanism or transcriptional activation mechanism (the “coding RNA component”), and (ii) products covered by the licensed patent rights (the “lipid products”). A product containing both a coding RNA component and a lipid product is referred to as a “licensed product.” Under the licensed patent rights, the Company is permitted to develop, manufacture and commercialize the licensed products for the delivery of coding RNA components to treat disease in humans. The Company has the right to grant sublicenses under this license. The patent rights licensed to the Company by MIT include claims that cover the Company’s customized lipid-based nanoparticles used for delivery of coding RNA components in its MRT platform and MRT5201. Under the license agreement, the Company is obligated to make annual license maintenance payments to MIT, payable on January 1 of each calendar year, of up to $0.2 million, which may be credited against royalties subsequently due on net sales of licensed products earned in the same calendar year. The Company paid no annual license maintenance fees to MIT during each of the three months ended June 30, 2019 and 2018 and paid $0.2 million and $0.1 million during the six months ended June 30, 2019 and 2018, respectively. The Company is also obligated to make milestone payments to MIT aggregating up to $1.375 million upon the achievement of specified clinical and regulatory milestones with respect to each licensed product and $1.250 million upon the Company’s first commercial sale of each licensed product, and to pay royalties of a low single-digit percentage to MIT based on the Company’s, and any of its affiliates’ and sublicensees’, net sales of licensed products. The royalties are payable on a product-by-product and country-by-country basis, and may be reduced in specified circumstances. The Company’s obligation to make royalty payments extends with respect to a licensed product in a country until the expiration of the last-to-expire patent or patent application licensed from MIT covering the licensed product in the country. In addition, the Company is obligated to pay MIT a low double-digit percentage of the portion of income from sublicensees that the Company ascribes to the MIT-licensed patents, excluding royalties on net sales and research support payments. Pursuant to such provision, the Company agreed to pay $0.7 million to MIT as MIT’s share of sublicense income with respect to the upfront payment received under the Sanofi Agreement. The amounts that the Company may owe to MIT will depend upon the relative value of the patents the Company licensed from MIT and sublicensed to Sanofi as compared to the other rights that the Company licensed to Sanofi. The determination of the relative value of such rights is subject to a process described in the Company’s license agreement with MIT (see Note 3). The agreement obligates the Company to use commercially reasonable efforts and expend a minimum amount of resources each year to develop licensed products in accordance with a development plan, and a development milestone timetable specified in the agreement; to use commercially reasonable efforts to commercialize licensed products; and upon commercialization, to make the licensed products reasonably available to the public. MIT has the right to terminate the agreement if the Company fails to pay amounts when due or otherwise materially breaches the agreement and fails to cure such nonpayment or breach within specified cure periods or in the event the Company ceases to carry on its business related to the agreement. In the event of a termination due to the Company’s breach caused by a due diligence failure of a licensed product, but where the Company has fulfilled its obligations with respect to a different licensed product, MIT may not terminate the agreement with respect to the different licensed product. MIT may immediately terminate the agreement if the Company or any of its affiliates brings specified patent challenges against MIT or assists others in bringing a patent challenge against MIT. The Company has the right to terminate the agreement for its convenience at any time on three months’ prior written notice to MIT and payment of all amounts due to MIT through the date of termination. The Company’s patent rights, and the rights of its affiliates and sublicensees, in specified licensed products may also terminate, if the Company, its affiliates or MIT receives a request from a third party to develop such licensed product for which the Company is unable to, within nine months of receiving notice of any such request, either demonstrate that the Company has initiated a fully funded project for the commercial development of such licensed product, and provide a business plan with acceptable milestones; demonstrate that the licensed product proposed by such third party would be competitive with a licensed product for which the Company has initiated a fully funded project; or enter into a sublicense agreement with such third party on commercially reasonable terms, and, in each case, MIT, in its sole discretion, grants a license to such third party for the specified patent rights. Research and development expenses related to this agreement totaled less than $0.1 million during each of the three months ended June 30, 2019 and 2018, and $0.1 million during each of the the six months ended June 30, 2019 and 2018, respectively. As of June 30, 2019 and December 31, 2018, there were no liabilities recorded by the Company related to this agreement. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of June 30, 2019 and December 31, 2018. Legal Proceedings The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities.

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2019
Related Party Transactions [Abstract]
Related Party Transactions	14. Related Party Transactions Consulting Agreement with Daniel S. Lynch In 2012, the Company entered into a consulting agreement with Daniel S. Lynch, the chairman of the Company’s board of directors, for the provision of consulting, advisory and related services. Pursuant to the consulting agreement, as amended through March 2015, Mr. Lynch was entitled to base compensation of $100,000 per year and was eligible to receive an annual performance bonus of up to 25% of his base compensation. In June 2018, the Company’s board of directors approved a director compensation program that became effective on the effective date of the registration statement related to the Company’s IPO. The Company has not made any payments to Mr. Lynch under the consulting agreement since the approval of the director compensation program. During the three and six months ended June 30, 2019 and 2018, the Company recorded general and administrative expenses of $0, $31,250, $0 and $0.1 million, respectively, related to this agreement. During the three and six months ended June 30, 2019 and 2018, the Company paid Mr. Lynch $0, $0.1 million, $11,250 and $0.1 million, respectively, in connection with his services provided under the agreement. As of June 30, 2019 and December 31, 2018, amounts due under this agreement totaled $0 and $11,250, respectively, which were included in accrued expenses on the consolidated balance sheets. Private Placement In connection with a private placement of the Company’s common stock, entities affiliated with Baupost Group, L.L.C., a substantial stockholder, purchased 2,352,941 shares of the Company’s common stock at a price per share of $8.50 for an aggregate purchase price of $20.0 million.

Subsequent Event

Subsequent Event	6 Months Ended
	Jun. 30, 2019
Subsequent Events [Abstract]
Subsequent Event	15. Subsequent Event On July 3, 2019, the Company filed a universal shelf registration statement on Form S-3 with the SEC (the “2019 Shelf”) to register for sale from time to time up to $250.0 million of common stock, preferred stock, debt securities, warrants and/or units in one or more offerings, which became effective on July 19, 2019 (File No. 333-232543). On July 3, 2019, the Company entered into an Open Market Sale Agreement SM

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Recently Adopted Accounting Pronouncement	Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (the “FASB”) issued ASU 2016-02, right-of-use 2016-02 The Company elected the permitted practical expedients within ASC 842, which allowed the Company to not reassess previous accounting conclusions around whether arrangements are or contain leases, and carried forward both the historical classification of leases and the treatment of initial direct costs. In addition, the Company elected to exclude leases with an initial term of one year or less in the recognized ROU assets and lease liabilities. Adoption of the new standard resulted in the recording of ROU assets and related lease liabilities of approximately $10.9 million and $13.0 million, respectively, as of January 1, 2019. The standard did not materially impact the Company’s consolidated net earnings and had no impact on cash flows. Refer to Note 12 for the additional disclosures required by ASC 842. The Company determines if an arrangement is a lease at inception. For leases where the Company is the lessee, ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent an obligation to make lease payments arising from the lease. ROU assets and lease liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. As most of the Company’s leases do not provide an implicit interest rate, the Company uses its incremental borrowing rate, which are the rates incurred to borrow on a collateralized basis over a term equal to the lease payments in a similar economic environment, in determining the present value of lease payments. The lease terms used to calculate the ROU asset and related lease liability include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for operating leases is recognized on a straight-line basis over the lease term as an operating expense. The Company has lease agreements which require payments for lease and non-lease In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception ( 2017-11” ) . 2017-11 In June 2018, the FASB issued ASU No. 2018-07, Compensation-Stock Compensation (Topic 718) 2018-07”), non-employees. non-employee 2018-07 non-employee
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In June 2016, the FASB issued guidance on the Measurement of Credit Losses on Financial Instruments. The guidance requires that credit losses be reported using an expected losses model rather than the incurred losses model that is currently used, and establishes additional disclosures related to credit risks. For available-for-sale In January 2017, the FASB issued ASU No. 2017-04, Intangibles—Goodwill and Other: Simplifying the Test for Goodwill Impairment (Topic 350) 2017-04”), 2017-04 In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808) 2018-18”). Revenue Recognition Collaborative Arrangements 2018-18

Sanofi Collaboration and Lice_2

Sanofi Collaboration and License Agreement (Tables)	6 Months Ended
	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Collaboration Revenue	The following table summarizes the Company’s collaboration revenue (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Collaboration revenue $ 1,174 $ — $ 2,648 $ —
Balance of Contract Liabilities Related to Collaboration Agreements	The following table presents the balance of the Company’s contract liabilities (in thousands): June 30, December 31, Contract liabilities Deferred revenue $ 43,207 $ 44,413

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	6 Months Ended
	Jun. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Definite-Lived Intangible Assets Subject to Amortization and Indefinite-Lived Intangible Assets	The tables below present the Company’s definite-lived intangible assets that are subject to amortization and indefinite-lived intangible assets: June 30, 2019 Estimated Gross Carrying Amount Accumulated Amortization Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 years $ 45,992 $ (1,212 ) $ 44,780 Indefinite-lived intangible assets: IPR&D—CF Indefinite 42,291 — 42,291 IPR&D—OTC Indefinite 18,559 — 18,559 Total intangible assets, net $ 106,842 $ (1,212 ) $ 105,630 December 31, 2018 Estimated Gross Carrying Amount Accumulated Amortization Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 years $ 45,992 $ (397 ) $ 45,595 Indefinite-lived intangible assets: IPR&D—CF Indefinite 42,291 — 42,291 IPR&D—OTC Indefinite 18,559 — 18,559 Total intangible assets, net $ 106,842 $ (397 ) $ 106,445

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis (in thousands): Fair Value Measurements as of June 30, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 17,554 $ — $ 17,554 U.S. government agency bonds — 71,961 — 71,961 $ — $ 89,515 $ — $ 89,515 Liabilities: Contingent consideration $ — $ — $ 120,233 $ 120,233 $ — $ — $ 120,233 $ 120,233 Fair Value Measurements as of December 31, 2018 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 23,318 $ — $ 23,318 U.S. government agency bonds — 88,904 — 88,904 $ — $ 112,222 $ — $ 112,222 Liabilities: Contingent consideration $ — $ — $ 103,642 $ 103,642 $ — $ — $ 103,642 $ 103,642
Schedule of Unobservable Inputs and Fair Value Components of Contingent Consideration	The following table presents the unobservable inputs and fair value of the components of the contingent consideration (dollar amounts in thousands): Unobservable Inputs at June 30, 2019 and December 31, 2018 Fair Value at Projected Year of Payment June 30, December 31, Earnout payments 2025 - 2039 $ 110,770 $ 94,999 Milestone payments 2025 - 2030 9,463 8,643 $ 120,233 $ 103,642
Schedule of Total Acquisition Related Contingent Consideration Liability	The following table presents a roll-forward of the total acquisition-related contingent consideration liability (in thousands): Fair Value Balance as of December 31, 2018 $ 103,642 Change in fair value of contingent consideration 16,591 Balance as of June 30, 2019 $ 120,233

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 consisted of the following (in thousands): June 30, December 31, Laboratory equipment $ 8,394 $ 7,012 Computer equipment 780 686 Office equipment 883 836 Leasehold improvements 5,634 5,635 Construction in progress 1,241 959 16,932 15,128 Less: Accumulated depreciation and amortization (6,029 ) (4,883 ) $ 10,903 $ 10,245

Accrued Expenses (Tables)

Accrued Expenses (Tables)	6 Months Ended
	Jun. 30, 2019
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): June 30, December 31, Accrued external research and development expenses $ 2,950 $ 1,901 Accrued employee compensation and benefits 2,392 2,933 Accrued consultant and professional fees 1,904 977 Other — 736 $ 7,246 $ 6,547

Incentive Stock Options and R_2

Incentive Stock Options and Restricted Stock (Tables)	6 Months Ended
	Jun. 30, 2019
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2018 (in thousands, except share and per share amounts): Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Intrinsic Value (in years) Outstanding as of December 31, 2018 6,236,006 $ 7.78 8.74 $ 1,104 Granted 2,614,650 $ 8.48 Exercised (221,401 ) $ 6.40 Forfeited (80,595 ) $ 7.85 Outstanding as of June 30, 2019 8,548,660 $ 8.03 8.83 $ 39,344 Exercisable as of June 30, 2019 2,671,285 $ 7.59 8.43 $ 13,472 Vested and expected to vest as of June 30, 2019 8,548,660 $ 8.03 8.83 $ 39,344 Exercisable as of December 31, 2018 1,827,004 $ 7.16 7.95 $ 632 Vested and expected to vest as of December 31, 2018 6,236,006 $ 7.78 8.74 $ 1,104
Summary of Restricted Stock Activity	The following table summarizes the Company’s restricted stock activity since December 31, 2018: Number of Shares Weighted Average Grant-Date Fair Value Unvested restricted common stock outstanding as of December 31, 2018 219,148 $ 1.27 Forfeited restricted common stock (1,334 ) $ 1.28 Vested restricted common stock (97,826 ) $ 1.27 Unvested restricted common stock outstanding as of June 30, 2019 119,988 $ 1.28
Summary of Stock-Based Compensation Expense	Stock-based compensation expense was classified in the condensed consolidated statements of operations as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Research and development expenses $ 1,284 $ 1,671 $ 2,153 $ 2,453 General and administrative expenses 2,265 1,204 3,356 1,805 $ 3,549 $ 2,875 $ 5,509 $ 4,258
Employee and Board of Directors [Member]
Summary of Assumptions Used in Black-Scholes Option-Pricing Model to Determine Grant-Date Fair Value of Stock Option Granted	The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees and directors: Six Months Ended June 30, 2019 2018 Risk-free interest rate 2.42 % 2.80 % Expected term (in years) 6.0 6.0 Expected volatility 73.3 % 75.7 % Expected dividend yield 0 % 0 %

Net Loss per Share (Tables)

Net Loss per Share (Tables)	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Summary of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Numerator: Net loss $ (27,832 ) $ (27,503 ) $ (61,030 ) $ (48,712 ) Accretion of redeemable convertible preferred stock to redemption value — (459 ) — (644 ) Net loss attributable to common stockholders $ (27,832 ) $ (27,962 ) $ (61,030 ) $ (49,356 ) Denominator: Weighted average common shares outstanding—basic and diluted 48,749,627 9,187,207 46,866,842 9,139,638 Net loss per share attributable to common stockholders—basic and diluted $ (0.57 ) $ (3.04 ) $ (1.30 ) $ (5.40 )
Schedule of Potential Securities Excluded from Computation of Earnings Per Share	The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: June 30, 2019 2018 Options to purchase common stock 8,548,660 6,093,408 Unvested restricted common stock 119,988 368,875 Redeemable convertible preferred stock (as converted to common stock) — 25,612,109 8,668,648 32,074,392

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Schedule of Lease Cost	The components of lease cost were as follows (dollar amounts in thousands): Three Months Ended Six Months Ended Lease Cost Operating lease cost $ 673 $ 1,346 Short-term lease cost — — Total lease cost $ 673 $ 1,346 Other Information Operating cash flows from operating leases $ 650 $ 1,282 Operating lease liabilities arising from obtaining right-of-use assets $ — $ — Weighted-average remaining lease term 9 years 9 years Weighted-average discount rate 17.5 % 17.5 %
Schedule of Future Minimum Rental Commitments for Operating Leases	As of June 30, 2019, minimum rental commitments under these leases are as follows (in thousands): June 30, 2019 2019 $ 1,301 2020 2,659 2021 2,737 2022 2,818 2023 2,860 2024 and thereafter 13,096 Total future minimum lease payments 25,471 Less: imputed interest (12,652 ) Present value of lease liabilities $ 12,819 As of December 31, 2018, minimum rental commitments under the real estate lease was as follows (in thousands): December 31, 2018 2019 $ 2,534 2020 2,610 2021 2,688 2022 2,769 2023 2,852 2024 and thereafter 13,096 Total future minimum lease payments $ 26,549

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	May 03, 2019USD ($)$ / sharesshares	Jul. 02, 2018USD ($)$ / sharesshares	Jun. 15, 2018	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Jun. 30, 2019USD ($)Subsidiary	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)Disease
Initial Public Offering [Line Items]
Number of wholly owned subsidiaries | Subsidiary								2
Reverse stock split description			one-for-5.5555 reverse stock split
Reverse stock split ratio			0.1800
Net proceeds from IPO		$ 113,200
Gross proceeds from issuance of private placement								$ 44,608
other estimated offering expenses									$ 2,421
Accumulated deficit				$ (307,233)				(307,233)		$ (246,203)
Net loss				(27,832)	$ (33,198)	$ (27,503)	$ (21,209)	(61,030)	$ (48,712)
Cash, cash equivalents and short-term investments				146,900				146,900
Sanofi Pasteur Collaboration and Licensing Agreement [Member] | Sanofi Pasteur Inc [Member]
Initial Public Offering [Line Items]
Number of infectious disease pathogens | Disease										5
Period of research term										3 years
Upfront payment received										$ 45,000
Sanofi Pasteur Collaboration and Licensing Agreement [Member] | Sanofi Pasteur Inc [Member] | Maximum [Member]
Initial Public Offering [Line Items]
Extended period of research term										8 years
Receivable from collaboration				$ 805,000				$ 805,000
IPO [Member]
Initial Public Offering [Line Items]
Issuance of common, shares | shares		9,714,371
Common stock issued and sold, per share | $ / shares		$ 3
Private Placement [Member]
Initial Public Offering [Line Items]
Issuance of common, shares | shares	5,582,940
Common stock issued and sold, per share | $ / shares	$ 8.50
Gross proceeds from issuance of private placement	$ 47,500
Placement agent fees	2,800
other estimated offering expenses	$ 600

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Indefinite-lived Intangible Assets [Line Items]
Right-of-use assets, net	$ 10,650
Operating lease, liability	$ 12,819
ASU 2016-02 [Member]
Indefinite-lived Intangible Assets [Line Items]
Right-of-use assets, net		$ 10,900
Operating lease, liability		$ 13,000

Sanofi Collaboration and Lice_3

Sanofi Collaboration and License Agreement - Additional Information (Detail) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2019USD ($)	Jun. 30, 2019USD ($)	Dec. 31, 2018USD ($)Disease
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Short-term Investments		$ 71,961	$ 88,904
Sanofi Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of infectious disease pathogens for vaccine development | Disease			5
Period of research term			3 years
Upfront payment received			$ 45,000
Reimbursable development costs payable period		60 days
Additional fee per added pathogen option			5,000
Maximum development and regulatory milestone payment receivable			63,000
Technology and process transfer milestone payment receivable			$ 10,000
Royalty payment term			10 years
Reduction in transaction price	$ 10,000
Non-refundable upfront payment	45,000
Estimated reimbursable employee cost	32,600
Estimated reimbursable development cost	54,500
Estimated milestone payments	$ 19,000
Revenue recognizing period		8 years
Revenue recognized from contract liabilities		$ 1,200
Sanofi Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Extended period of research term			8 years
Receivable from collaboration			$ 805,000
Sales milestone payment receivable			$ 85,000

Sanofi Collaboration and Lice_4

Sanofi Collaboration and License Agreement - Summary of Collaboration Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration revenue	$ 1,174	$ 2,648

Sanofi Collaboration and Lice_5

Sanofi Collaboration and License Agreement - Balance of Contract Liabilities Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Contract liabilities
Deferred revenue	$ 43,207	$ 44,413

Intangible Assets and Goodwil_2

Intangible Assets and Goodwill - Summary of Definite-Lived Intangible Assets Subject to Amortization and Indefinite-Lived Intangible Assets (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, accumulated amortization	$ (1,212)	$ (397)
Total intangible assets, gross carrying amount	106,842	106,842
Total intangible assets, net carrying amount	$ 105,630	$ 106,445
In-Process Research and Development [Member] | MRT Product [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, estimated life	8 years	8 years
Definite-lived intangible assets, gross carrying amount	$ 45,992	$ 45,992
Definite-lived intangible assets, accumulated amortization	(1,212)	(397)
Definite-lived intangible assets, net carrying amount	44,780	45,595
In-Process Research and Development [Member] | Cystic Fibrosis [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross carrying amount/net carrying amount	42,291	42,291
In-Process Research and Development [Member] | OTC Deficiency [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross carrying amount/net carrying amount	$ 18,559	$ 18,559

Intangible Assets and Goodwil_3

Intangible Assets and Goodwill - Additional Information (Detail) - USD ($)	Jan. 01, 2019	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Finite Lived Intangible Assets [Line Items]
Impairment of Intangible Assets, Finite-lived		$ 0
Goodwill		21,359,000	$ 21,359,000	$ 21,359,000
Goodwill changes			0
Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Goodwill		21,400,000	$ 21,400,000
Shire's MRT Program [Member] | Sanofi Agreement [Member]
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, estimated life			8 years
Amortization of Intangible Assets		300,000	$ 800,000
Estimated amortization expense of intangible assets for 2019		3,600,000	3,600,000
Estimated amortization expense of intangible assets for 2020		9,000,000	9,000,000
Estimated amortization expense of intangible assets for 2021		9,900,000	9,900,000
Estimated amortization expense of intangible assets for 2022		4,200,000	4,200,000
Estimated amortization expense of intangible assets for 2023		$ 2,300,000	$ 2,300,000
OTC Deficiency Program [Member]
Finite Lived Intangible Assets [Line Items]
Amortization of Intangible Assets	$ 0

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities - Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Total, assets	$ 89,515	$ 112,222
Liabilities:
Total, liabilities	120,233	103,642
Fair Value, Inputs, Level 2
Assets:
Total, assets	89,515	112,222
Fair Value, Inputs, Level 3
Liabilities:
Total, liabilities	120,233	103,642
Money Market Funds
Assets:
Total, assets	17,554	23,318
Money Market Funds | Fair Value, Inputs, Level 2
Assets:
Total, assets	17,554	23,318
U.S. Government Agency Bonds
Assets:
Total, assets	71,961	88,904
U.S. Government Agency Bonds | Fair Value, Inputs, Level 2
Assets:
Total, assets	71,961	88,904
Contingent Consideration
Liabilities:
Total, liabilities	120,233	103,642
Contingent Consideration | Fair Value, Inputs, Level 3
Liabilities:
Total, liabilities	$ 120,233	$ 103,642

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2018USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 74,904,000	$ 55,199,000	$ 16,495,000
Short-term investments amortized cost	71,400,000
Short-term investments unrealized gain	600,000
Short-term investments fair value	$ 72,000,000
Contingent Consideration
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Discount Rate	13.5	14.5
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short term investments maturity period	1 year
Money Market Funds | Fair Value, Inputs, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 17,600,000	$ 23,300,000
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value,assets transfers from Level 1 to Level 2	0	0
Fair value,assets transfers from Level 2 to Level 1	0	0
Fair Value, assets transfers into (out of) Level 3	0	0
Fair value, liabilities transfers from Level 1 to Level 2	0	0
Fair value, liabilities transfers from Level 2 to Level 1	0	0
Fair Value, liabilities transfers into (out of) Level 3	$ 0	$ 0

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities - Schedule of Unobservable Inputs and Fair Value Components of Contingent Consideration (Detail) - Contingent Consideration - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	$ 120,233	$ 103,642
Earnout Payments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	$ 110,770	$ 94,999
Earnout Payments | Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2025	2025
Earnout Payments | Maximum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2039	2039
Milestone Payments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	$ 9,463	$ 8,643
Milestone Payments | Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2025	2025
Milestone Payments | Maximum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2030	2030

Fair Value of Financial Asset_6

Fair Value of Financial Assets and Liabilities - Schedule of Total Acquisition Related Contingent Consideration Liability (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Business Combination, Contingent Consideration, Liability [Abstract]
Beginning Balance			$ 103,642
Change in fair value of contingent consideration	$ 4,889	$ 7,852	16,591	$ 12,760
Ending Balance	$ 120,233		$ 120,233

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 16,932	$ 15,128
Less: Accumulated depreciation and amortization	(6,029)	(4,883)
Property and equipment, net	10,903	10,245
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	8,394	7,012
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	780	686
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	883	836
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,634	5,635
Construction In Progress [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,241	$ 959

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 0.6	$ 0.9	$ 1.1	$ 1.4

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 2,950	$ 1,901
Accrued employee compensation and benefits	2,392	2,933
Accrued consultant and professional fees	1,904	977
Other		736
Total accrued expenses	$ 7,246	$ 6,547

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock - Additional Information (Detail) - shares	Jun. 30, 2019	Dec. 31, 2018	Jul. 02, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Preferred Shares Issued	0	0
Preferred Shares Outstanding	0	0
Conversion of redeemable convertible preferred stock to common stock, Shares			25,612,109
Preferred Shares Authorized	0	0
Convertible Preferred Stock Subject to Mandatory Redemption
Temporary Equity [Line Items]
Preferred Shares Issued				142,288,292
Preferred Shares Outstanding				142,288,292

Incentive Stock Options and R_3

Incentive Stock Options and Restricted Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2019	Jun. 15, 2018	Mar. 07, 2018	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense				$ 847	$ 847
Common stock, shares issued				51,010,368	51,010,368		45,139,955
Unrecognized compensation cost related to unvested stock-based awards				$ 29,300	$ 29,300
Unrecognized compensation cost, period for recognition					2 years 9 months
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Intrinsic value of stock options, exercised					$ 600	$ 300
Weighted average grant-date fair value					$ 5.56	$ 5.91
2018 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares reserved for issuance				4,765,663	4,765,663
Number of shares remaining available for issuance			360,514
Increase in common stock issued	1,805,598
Percentage threshold of outstanding shares under the plan		4.00%
Percentage of exercise price per share of fair market value		100.00%
Expiration period of stock options after grant date		10 years
2018 Stock Incentive Plan [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock issued during period under the plan		3,349,582
Stock options for purchase of common stock held, exercisable period		1 year
2018 Stock Incentive Plan [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options for purchase of common stock held, exercisable period		4 years
2018 Stock Incentive Plan [Member] | Share-based Compensation Award, Tranche One [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares reserved for issuance			2,512,187
2018 Stock Incentive Plan [Member] | Share-based Compensation Award, Tranche Two [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares reserved for issuance			1,013,167
2018 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares reserved for issuance	870,096		418,697
Increase in common stock issued	451,399
Percentage threshold of outstanding shares under the plan			1.00%
Common stock, shares issued				0	0
2018 Employee Stock Purchase Plan [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock issued during period under the plan			837,395
2016 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Sharebased Compensation Arrangement by Sharebased Payment Award Shares Cancelled in Period					87,364
Percentage of exercise price per share of fair market value		100.00%
Expiration period of stock options after grant date		10 years
Stock options for purchase of common stock held, exercisable period		4 years
Former Employee [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Issue of shares to former employee satisfaction claims					67,406
Former Employee [Member] | General and Administrative Expense [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense				$ 800	$ 800

Incentive Stock Options and R_4

Incentive Stock Options and Restricted Stock - Summary of Stock Option Activity (Detail) - Stock Options [Member] $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares
Number of shares
Number of shares, beginning balance | shares	6,236,006
Number of shares, granted | shares	2,614,650
Number of shares, exercised | shares	(221,401)
Number of shares, forfeited | shares	(80,595)
Number of shares, ending balance | shares	8,548,660	6,236,006
Number of shares, exercisable | shares	2,671,285	1,827,004
Number of shares, vested and expected to vest, ending balance | shares	8,548,660	6,236,006
Weighted average exercise price
Weighted average exercise price, beginning balance | $ / shares	$ 7.78
Weighted average exercise price, granted | $ / shares	8.48
Weighted average exercise price, exercised | $ / shares	6.40
Weighted average exercise price, forfeited | $ / shares	7.85
Weighted average exercise price, ending balance | $ / shares	8.03	$ 7.78
Weighted average exercise price, exercisable | $ / shares	7.59	7.16
Weighted average exercise price, vested and expected to vest, ending balance | $ / shares	$ 8.03	$ 7.78
Weighted average remaining contractual term
Weighted average remaining contractual term, ending balance	8 years 9 months 29 days	8 years 8 months 26 days
Weighted average remaining contractual term, exercisable	8 years 5 months 4 days	7 years 11 months 12 days
Weighted average remaining contractual term, vested and expected to vest, ending balance	8 years 9 months 29 days	8 years 8 months 26 days
Intrinsic value
Intrinsic value, ending balance | $	$ 39,344	$ 1,104
Intrinsic value, exercisable | $	13,472	632
Intrinsic value, vested and expected to vest, ending balance | $	$ 39,344	$ 1,104

Incentive Stock Option and Rest

Incentive Stock Option and Restricted Stock - Summary of Assumptions Used in Black-Scholes Option-Pricing Model to Determine Grant-Date Fair Value of Stock Option Granted (Detail) - Employees and Directors [Member] - Stock Options [Member]	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.42%	2.80%
Expected term (in years)	6 years	6 years
Expected volatility	73.30%	75.70%
Expected dividend yield	0.00%	0.00%

Incentive Stock Option and Re_2

Incentive Stock Option and Restricted Stock - Summary of Restricted Stock Activity (Detail) - Restricted Common Stock [Member]	6 Months Ended
	Jun. 30, 2019$ / sharesshares
Number of shares
Number of shares, beginning balance | shares	219,148
Number of shares, forfeited | shares	(1,334)
Number of shares, vested | shares	(97,826)
Number of shares, ending balance | shares	119,988
Weighted average grant-date fair value
Weighted average grant-date fair value, beginning balance | $ / shares	$ 1.27
Weighted average grant-date fair value, forfeited | $ / shares	1.28
Weighted average grant-date fair value, vested | $ / shares	1.27
Weighted average grant-date fair value, ending balance | $ / shares	$ 1.28

Incentive Stock Option and Re_3

Incentive Stock Option and Restricted Stock - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Stock-based compensation expense	$ 3,549	$ 2,875	$ 5,509	$ 4,258
Research and Development Expenses [Member]
Stock-based compensation expense	1,284	1,671	2,153	2,453
General and Administrative Expenses [Member]
Stock-based compensation expense	$ 2,265	$ 1,204	$ 3,356	$ 1,805

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Benefit from income taxes	$ 1,500	$ 486	$ 2,602

Net Loss per Share - Summary of

Net Loss per Share - Summary of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (27,832)	$ (33,198)	$ (27,503)	$ (21,209)	$ (61,030)	$ (48,712)
Accretion of redeemable convertible preferred stock to redemption value			(459)			(644)
Net loss attributable to common stockholders	$ (27,832)		$ (27,962)		$ (61,030)	$ (49,356)
Denominator:
Weighted average common shares outstanding—basic and diluted	48,749,627		9,187,207		46,866,842	9,139,638
Net loss per share attributable to common stockholders—basic and diluted	$ (0.57)		$ (3.04)		$ (1.30)	$ (5.40)

Net Loss per Share - Additional

Net Loss per Share - Additional Information (Detail) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net loss per share attributable to common stockholders—basic and diluted	$ (0.57)	$ (3.04)	$ (1.30)	$ (5.40)
Antidilutive securities excluded from computation of basic net loss per share			8,668,648	32,074,392
Restricted Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share			119,988	368,875
Restricted Common Stock [Member] | Weighted Average [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	147,914	412,040	172,941	451,286
Previously Reported [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net loss per share attributable to common stockholders—basic and diluted		$ 2.94		$ 5.26

Net Loss per Share - Summary _2

Net Loss per Share - Summary of Potential Common Shares Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	8,668,648	32,074,392
Stock Options [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	8,548,660	6,093,408
Restricted Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	119,988	368,875
Redeemable Convertible Preferred Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share		25,612,109

Leases - Additional Information

Leases - Additional Information (Details) $ in Millions	1 Months Ended		3 Months Ended	6 Months Ended
	Mar. 31, 2018	Jun. 30, 2017USD ($)ft²	Jun. 30, 2018USD ($)	Jun. 30, 2019Lease	Jun. 30, 2018USD ($)
Lessee Lease Description [Line Items]
Number of operating leases | Lease				2
Remaining term of lease				9 years
Rent expense			$ 0.8		$ 1.7
Lexington, Massachusetts [Member]
Lessee Lease Description [Line Items]
Office and laboratory space | ft²		59,000
Lease, term of contract	5 years	10 years
Monthly lease payments		$ 0.2
Lease expiration period	Mar. 31, 2023			Apr. 30, 2028
Percentage of annual increase in operating lease		3.00%
Cash deposit collateral		$ 1
Lexington, Massachusetts [Member] | Maximum [Member]
Lessee Lease Description [Line Items]
Remaining term of lease	9 years

Leases - Schedule of Lease Cost

Leases - Schedule of Lease Cost (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019USD ($)	Jun. 30, 2019USD ($)
Lease Cost
Operating lease cost	$ 673	$ 1,346
Total lease cost	673	1,346
Operating cash flows from operating leases	$ 650	$ 1,282
Weighted-average remaining lease term	9 years	9 years
Weighted-average discount rate	17.50%	17.50%

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Rental Commitments for Operating Leases (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Leases [Abstract]
2019	$ 1,301	$ 2,534
2020	2,659	2,610
2021	2,737	2,688
2022	2,818	2,769
2023	2,860	2,852
2024 and thereafter	13,096	13,096
Total future minimum lease payments	25,471	$ 26,549
Less: imputed interest	(12,652)
Present value of lease liabilities	$ 12,819

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Loss Contingencies [Line Items]
Research and development	$ 16,625,000	$ 15,219,000	$ 34,048,000	$ 27,921,000
Research agreement, payable amount	2,950,000		$ 2,950,000		$ 1,901,000
Roche Diagnostics Corporation Master Supply Agreement [Member]
Loss Contingencies [Line Items]
Raw material to purchase as percentage of demand			80.00%
Agreement extended date			Dec. 31, 2024
Commitment amount			$ 20,400,000
Purchase commitments, year 2019	6,000,000		6,000,000
Purchase commitments, year 2020	500,000		500,000
Purchase commitments, year 2021	3,500,000		3,500,000
Purchase commitments, year 2022	3,500,000		3,500,000
Purchase commitments, year 2023	3,500,000		3,500,000
Purchase commitments, year 2024	3,500,000		3,500,000
Research and development	1,000,000	800,000	3,500,000	1,700,000
MIT Research Agreement [Member]
Loss Contingencies [Line Items]
Research and development	200,000	300,000	500,000	600,000
Research agreement, committed amount			$ 3,100,000
Research agreement, expiration period			2019-10
Research agreement, payment			$ 3,100,000		2,500,000
Research agreement, payable amount	0		0		0
MIT Exclusive Patent License Agreement [Member]
Loss Contingencies [Line Items]
Research and development	100,000	100,000	100,000	100,000
Research agreement, total committed amount	0		0		$ 0
Annual license maintenance payments	200,000		200,000
Payments of annual license maintenance fees	0	$ 0	200,000	$ 100,000
MIT Exclusive Patent License Agreement [Member] | Sanofi Pasteur Inc [Member]
Loss Contingencies [Line Items]
Upfront payment received	$ 700,000		700,000
MIT Exclusive Patent License Agreement [Member] | Milestone Payment One [Member]
Loss Contingencies [Line Items]
License agreement, milestone payments			1,375,000
MIT Exclusive Patent License Agreement [Member] | Milestone Payment Two [Member]
Loss Contingencies [Line Items]
License agreement, milestone payments			$ 1,250,000

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2012	May 03, 2019	Dec. 31, 2018
Board of Directors Chairman [Member] | Consulting Agreement [Member]
Related Party Transaction [Line Items]
Base compensation per year					$ 100,000
General and administrative expenses	$ 0	$ 31,250	$ 0	$ 100,000
Amount paid for services	0	$ 100,000	11,250	$ 100,000
Amount due to related party	$ 0		$ 0				$ 11,250
Board of Directors Chairman [Member] | Consulting Agreement [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Annual performance bonus					25.00%
Private Placement [Member]
Related Party Transaction [Line Items]
Sale of Stock, Price Per Share						$ 8.50
Private Placement [Member] | Baupost Group LLC [Member]
Related Party Transaction [Line Items]
Sale of Stock, Number of Shares Issued in Transaction			2,352,941
Sale of Stock, Price Per Share	$ 8.50		$ 8.50
Sale of Stock, Consideration Received on Transaction			$ 20,000,000

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) - Subsequent Event [Member] $ in Millions	Jul. 03, 2019USD ($)
Subsequent Event [Line Items]
Sale from time to time of equity and debt securities	$ 250
Open Market Sale Agreement [Member]
Subsequent Event [Line Items]
Market sale aggreement of common stock	$ 50