Telesis Bio 10-Q 6 Nov 19 2019 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2019	Nov. 04, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
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		60,022,067

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 98,930	$ 55,199
Short-term investments	111,233	88,904
Prepaid expenses and other current assets	6,648	4,474
Restricted cash	950	1,025
Total current assets	217,761	149,602
Property and equipment, net	10,680	10,245
Right-of-use assets, net	10,527
Goodwill	21,359	21,359
Intangible assets, net	86,695	106,445
Other long-term assets	2,492
Total assets	349,514	287,651
Current liabilities:
Accounts payable	3,407	5,168
Accrued expenses	8,767	6,547
Current portion of deferred revenue	12,603	2,572
Current portion of operating lease liability	489
Total current liabilities	25,266	14,287
Long-term portion of contingent consideration	100,399	103,642
Deferred revenue, net of current portion	30,046	41,841
Deferred tax liabilities		481
Deferred rent		2,105
Operating lease liability, net of current portion	12,230
Total liabilities	167,941	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 September 30, 2019 and December 31, 2018; no shares issued and outstanding as of September 30, 2019 and December 31, 2018
Common stock, $0.001 par value; 200,000,000 shares authorized as of September 30, 2019 and December 31, 2018; 60,020,725 shares and 45,139,955 shares issued and outstanding as of September 30, 2019 and December 31, 2018, respectively	60	45
Additional paid-in capital	509,294	371,257
Accumulated deficit	(328,460)	(246,203)
Accumulated other comprehensive income	679	196
Total stockholders' equity (deficit)	181,573	125,295
Total liabilities and stockholders' equity (deficit)	$ 349,514	$ 287,651

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 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	60,020,725	45,139,955
Common stock, shares outstanding	60,020,725	45,139,955

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Collaboration revenue	$ 1,266	$ 238	$ 3,914	$ 238
Operating expenses:
Research and development	17,295	12,933	51,343	40,854
General and administrative	6,881	5,957	21,284	16,726
Change in fair value of contingent consideration	(19,834)	26,829	(3,243)	39,589
Impairment of intangible asset	18,559		18,559
Total operating expenses	22,901	45,719	87,943	97,169
Loss from operations	(21,635)	(45,481)	(84,029)	(96,931)
Other income (expense):
Interest income	428	318	1,306	499
Other expense	(20)	(7)	(20)	(52)
Total other income (expense), net	408	311	1,286	447
Loss before benefit from income taxes	(21,227)	(45,170)	(82,743)	(96,484)
Benefit from income taxes	0	2,524	486	5,126
Net loss	(21,227)	(42,646)	(82,257)	(91,358)
Accretion of redeemable convertible preferred stock to redemption value				(644)
Net loss attributable to common stockholders	$ (21,227)	$ (42,646)	$ (82,257)	$ (92,002)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.41)	$ (0.97)	$ (1.69)	$ (5.13)
Weighted average common shares outstanding—basic and diluted	51,891,157	44,036,206	48,574,275	17,949,026

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (21,227)	$ (42,646)	$ (82,257)	$ (91,358)
Other comprehensive income (loss):
Unrealized gains (losses) on available-for-sale securities, net of tax of $0	109	(76)	483	(156)
Comprehensive loss	$ (21,118)	$ (42,722)	$ (81,774)	$ (91,514)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 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, ending balance at Mar. 31, 2018						$ 193,081
Redeemable convertible preferred stock, shares outstanding, ending balance at Mar. 31, 2018						142,288,292
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	(156)
Net loss	(91,358)
Ending balances at Sep. 30, 2018	129,417	$ 45	369,614	(240,166)	(76)
Ending balance, shares at Sep. 30, 2018		45,142,090
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
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)
Exercise of stock options	278		278
Exercise of stock options, Shares		49,459
Forfeited restricted common stock, Shares		(311)
Redeemable Convertible Preferred Stock, ending balance at Jun. 30, 2018						$ 193,540
Redeemable convertible preferred stock, shares outstanding, ending balance at Jun. 30, 2018						142,288,292
Ending balances at Jun. 30, 2018	(138,414)	$ 10	59,096	(197,520)
Ending balance, shares at Jun. 30, 2018		9,631,939
Unrealized gains on available-for-sale securities	(76)				(76)
Stock-based compensation expense	1,434		1,434
Net loss	(42,646)			(42,646)
Issuance of common stock in connection with public offering, net of underwriting discounts and commissions and offering costs	113,192	$ 9	113,183
Issuance of common stock in connection with public offering, net of underwriting discounts and commissions and offering costs(shares)		9,714,371
Conversion of redeemable convertible preferred stock to common stock	193,540	$ 26	193,514			$ (193,540)
Conversion of redeemable convertible preferred stock to common stock, Shares		25,612,109				(142,288,292)
Issuance of common stock in full settlement of contingent consideration anti-dilution liability	2,387		2,387
Issuance of common stock in full settlement of contingent consideration anti-dilution liability(shares)		183,619
Issuance of common stock due to the aggregation of fractional shares in connection with reverse stock split(shares)		52
Ending balances at Sep. 30, 2018	129,417	$ 45	369,614	(240,166)	(76)
Ending balance, shares at Sep. 30, 2018		45,142,090
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
Unrealized gains on available-for-sale securities	$ 155				155
Stock-based compensation expense	1,959		1,959
Net loss	(33,198)			(33,198)
Exercise of stock options	897		897
Exercise of stock options, Shares		154,484
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
Unrealized gains on available-for-sale securities	$ 483
Net loss	(82,257)
Issuance of common stock in connection with a former employee letter agreement (Note 9)	$ 847
Redeemable convertible preferred stock, shares outstanding, ending balance at Sep. 30, 2019	0
Ending balances at Sep. 30, 2019	$ 181,573	$ 60	509,294	(328,460)	679
Ending balance, shares at Sep. 30, 2019		60,020,725
Beginning balances at Mar. 31, 2019	95,108	$ 45	374,113	(279,401)	351
Beginning balance, Shares at Mar. 31, 2019		45,294,439
Unrealized gains on available-for-sale securities	219				219
Stock-based compensation expense	2,703		2,703
Net loss	(27,832)			(27,832)
Exercise of stock options	519		519
Exercise of stock options, Shares		66,917
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 private placement, net of placement agent fees and offering costs,shares		5,582,940
Issuance of common stock in connection with a former employee letter agreement (Note 9)	847		847
Issuance of common stock in connection with a former employee letter agreement (Note 9), Shares		67,406
Forfeited restricted common stock	(1)		(1)
Forfeited restricted common stock, Shares		(1,334)
Ending balances at Jun. 30, 2019	115,697	$ 51	422,309	(307,233)	570
Ending balance, shares at Jun. 30, 2019		51,010,368
Unrealized gains on available-for-sale securities	109				109
Stock-based compensation expense	2,899		2,899
Net loss	(21,227)			(21,227)
Exercise of stock options	84		84
Exercise of stock options, Shares		10,806
Issuance of common stock in connection with public offering, net of underwriting discounts and commissions and offering costs	$ 84,011	$ 9	84,002
Issuance of common stock in connection with public offering, net of underwriting discounts and commissions and offering costs(shares)		9,000,000
Forfeited restricted common stock, Shares		(449)
Redeemable convertible preferred stock, shares outstanding, ending balance at Sep. 30, 2019	0
Ending balances at Sep. 30, 2019	$ 181,573	$ 60	$ 509,294	$ (328,460)	$ 679
Ending balance, shares at Sep. 30, 2019		60,020,725

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash flows - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (21,227)	$ (82,257)	$ (91,358)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense		2,951	1,923
Stock-based compensation expense		8,408	5,693
Impairment of intangible asset	18,559	18,559
Change in fair value of contingent consideration	(19,834)	(3,243)	39,589
Deferred income tax benefit		(486)	(5,126)
Changes in operating assets and liabilities:
Prepaid expenses and other assets		(4,785)	(1,602)
Right-of-use assets		357
Accounts payable		(1,740)	(1,593)
Accrued expenses		2,244	1,394
Deferred rent			429
Lease liability		(270)
Deferred revenue		(1,638)	44,762
Net cash used in operating activities		(61,900)	(5,889)
Cash flows from investing activities:
Purchases of investments		(138,156)	(128,313)
Sales and maturities of investments		116,311	15,998
Purchases of property and equipment		(2,241)	(5,567)
Net cash used in investing activities		(24,086)	(117,882)
Cash flows from financing activities:
Proceeds from public offering, net of underwriting discounts and commissions		84,600
Payments of public offering costs		(589)
Proceeds from private placement, net of placement agent fees		44,608
Payments of private placement offering costs		(477)
Proceeds from initial public offering of common stock, net of underwriting discounts and commissions			117,447
Payments of initial public offering costs			(4,089)
Proceeds from option exercises		1,500	278
Net cash provided by financing activities		129,642	113,636
Net increase (decrease) in cash, cash equivalents and restricted cash		43,656	(10,135)
Cash, cash equivalents and restricted cash at beginning of period		56,224	50,024
Cash, cash equivalents and restricted cash at end of period	99,880	99,880	39,889
Cash, cash equivalents and restricted cash at end of period:
Cash and cash equivalents	98,930	98,930	38,864
Restricted cash	950	950	1,025
Total cash, cash equivalents and restricted cash at end of period	$ 99,880	99,880	39,889
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses		2	363
Deferred offering costs included in accounts payable and accrued expenses			13
Issuance of common stock in connection with acquisition of MRT Program			2,387
Issuance of common stock in connection with a former employee letter agreement (Note 9)		$ 847
Accretion of redeemable convertible preferred stock to redemption value			$ 644

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 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 primarily focused on applying our MRT platform to treat pulmonary diseases caused by insufficient protein production or where production of proteins can modify disease. The Company is developing 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 pre-defined one-month mid-dose 1 mid-dose In September 2019, the Company announced its decision to discontinue the development of MRT5201, a liver targeted treatment for ornithine transcarbamylase (“OTC”) deficiency. The Company’s decision to discontinue the development of MRT5201 for OTC deficiency was based on data from recently completed preclinical studies which did not support the desired pharmacokinetic and safety profile for advancement of the program. 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 condensed 10-K 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 September 30, 2019, the results of its operations for the three and nine months ended September 30, 2019 and 2018, and its cash flows for the nine months ended September 30, 2019 and 2018. The financial data and other information disclosed in these notes related to the three and nine months ended September 30, 2019 and 2018 are also unaudited. The results for the three and nine months ended September 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 program. Reverse Stock Split On June 15, 2018, the Company effected a one-for-5.5555 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. On July 3, 2019, the Company filed a universal shelf registration statement on Form S-3 No. 333-232543). On July 3, 2019, the Company entered into an Open Market Sale Agreement SM 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 three 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 September 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, the proceeds from a private placement of the Company’s common stock and the proceeds from a public offering of the Company’s common stock. The Company has incurred recurring losses and cash outflows from operations since its inception, including net losses of $82.3 million and $91.4 million for the nine months ended September 30, 2019 and 2018, respectively. In addition, the Company had an accumulated deficit of $328.5 million as of September 30, 2019. The Company expects to continue to generate operating losses for the foreseeable future. As of November 6 o the first half of 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.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 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. No. 2016-02, Leases (Topic 842) 2016-02”), Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (the “FASB”) issued ASU 2016-02, right-of-use 2016-02 Accounting Standards Codification (“ASC”) . Subsequent to the issuance of Topic , the FASB clarified the guidance through several ASUs; hereinafter the collection of lease guidance is referred to as “ASC ”. 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 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 – 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 - 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 Licens

Sanofi Collaboration and License Agreement	9 Months Ended
	Sep. 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 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 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 Under the terms of the Sanofi Agreement, the Company has granted to Sanofi exclusive, worldwide licenses under applicable patents, patent applications, know-how 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, 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 certain non-clinical 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 country-by-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 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 non-clinical ; non-cash Under ASC 606, at the end of each reporting period, the Company re-evaluates non-refundable million in reimbursable development costs including out-of-pocket costs paid to third parties and manufacturing costs and an estimated $ million in milestone payments . Under ASC 606, the Company recognized revenue using the cost-to-cost cost-to-cost The following table summarizes the Company’s collaboration revenue (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Collaboration revenue $ 1,266 $ 238 $ 3,914 $ 238 The following table presents the balance of the Company’s contract liabilities (in thousands): September 30, December 31, Contract liabilities Deferred revenue $ 42,649 $ 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 Included in prepaid expenses and other current assets on the condensed consolidated balance sheets as of September 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	9 Months Ended
	Sep. 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: September 30, 2019 Estimated Life Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 $ 45,992 $ (1,588 ) $ — $ 44,404 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 $ (1,588 ) $ (18,559 ) $ 86,695 December 31, 2018 Estimated Life Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 $ 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 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 nine months ended September 30, 2019, the Company recorded amortization expense of $0.4 million and $1.2 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. The Company determined that the discontinuation of the development of MRT5201 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 will not be investing any additional funds to this program and has reallocated all resources previously dedicated to the OTC deficiency , 18.6 million in the three and nine months ended September , , representing the entire value of the indefinite-lived IPR&D related to the OTC deficiency program. Additionally, the Company removed the contingent consideration liability related to this program as of September , (see Note . As a result of the termination of the planned Phase clinical trial for MRT in patients with OTC deficiency, the Company has recorded $ million in short-term receivables, which is included in prepaid expenses and other current assets on the condensed consolidated balance sheet as of September , , related to refundable advance payments for this program with one of the Company’s contract research organizations. The research and development expenses related to non-refundable had $ million included in accrued expenses for incurred costs related to the OTC deficiency program, which is expected to be paid by the end of the first quarter of 2020. 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 nine months ended September 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	9 Months Ended
	Sep. 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 September 30, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 48,872 $ — $ 48,872 U.S. government agency bonds — 111,233 — 111,233 $ — $ 160,105 $ — $ 160,105 Liabilities: Contingent consideration $ — $ — $ 100,399 $ 100,399 $ — $ — $ 100,399 $ 100,399 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 nine months ended September 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 September 30, 2019 and December 31, 2018 consisted of money market funds totaling $48.9 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 September 30, 2019 and December 31, 2018 consisted of U.S. government agency bonds and were classified as available-for-sale one 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 to Shire contingent upon the achievement of potential future milestones and earnout payments. 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 September 30, 2019 and December 31, Fair Value at Projected Year of Payment September 30, December 31, Earnout payments 2026 - 2039 $ 93,078 $ 94,999 Milestone payments 2026 - 2030 7,321 8,643 $ 100,399 $ 103,642 The discount rate used in the third-party valuation was 13.5% and 14.5% as of September 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 Discontinuation of MRT5201 (23,174 ) Increase 19,931 Balance as of September 30, 2019 $ 100,399 The n et

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 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): September 30, December 31, Laboratory equipment $ 8,788 $ 7,012 Computer equipment 780 686 Office equipment 883 836 Leasehold improvements 5,634 5,635 Construction in progress 1,237 959 17,322 15,128 Less: Accumulated depreciation and amortization (6,642 ) (4,883 ) $ 10,680 $ 10,245 Depreciation and amortization expense related to property and equipment was $0.6 million, $0.5 million, $1.8 million and $1.9 million for the three months ended September 30, 2019 and 2018 and the nine months ended September 30, 2019 and 2018, respectively.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2019
Payables and Accruals [Abstract]
Accrued Expenses	7. Accrued Expenses Accrued expenses consisted of the following (in thousands): September 30, December 31, Accrued external research and development expenses $ 4,221 $ 1,901 Accrued employee compensation and benefits 3,283 2,933 Accrued consultant and professional fees 1,263 977 Other — 736 $ 8,767 $ 6,547

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	9 Months Ended
	Sep. 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 September 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	9 Months Ended
	Sep. 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 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 September 30, 2019, a total of 142,262 shares issued under the 2016 plan have been cancelled for a total of 4,820,561 shares of common stock reserved for issuance under this plan as of September 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 September 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 2,723,200 $ 8.52 Exercised (232,207 ) $ 6.47 Forfeited (158,493 ) $ 8.02 Outstanding as of September 30, 2019 8,568,506 $ 8.04 8.67 $ 16,352 Exercisable as of September 30, 2019 3,071,842 $ 7.65 8.21 $ 6,970 Vested and expected to vest as of September 30, 2019 8,568,506 $ 8.04 8.67 $ 16,352 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 nine months ended September 30, 2019 and 2018 was $0.6 million and $0.2 million, respectively. The weighted average grant-date fair value per share of stock options granted was $5.58 and $5.97 during the nine months ended September 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 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: Nine Months Ended September 30, 2019 2018 Risk-free interest rate 2.39 % 2.81 % Expected term (in years) 6.0 6.0 Expected volatility 73.1 % 75.6 % 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 (1,783 ) $ 1.28 Vested restricted common stock (144,688 ) $ 1.27 Unvested restricted common stock outstanding as of September 30, 2019 72,677 $ 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 Nine Months Ended 2019 2018 2019 2018 Research and development expenses $ 1,301 $ 639 $ 3,454 $ 3,092 General and administrative expenses 1,598 796 4,954 2,601 $ 2,899 $ 1,435 $ 8,408 $ 5,693 Included in general and administrative stock-based compensation expense during the nine months ended September 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 September 30, 2019, total unrecognized compensation cost related to the unvested stock-based awards was $26.7 million, which is expected to be recognized over a weighted average period of 2.57 years.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes The Company recognized an income tax benefit of $0 and $2.5 million during the three months ended September 30, 2019 and 2018, respectively. The Company recognized an income tax benefit of $0.5 million and $5.1 million during the nine months ended September 30, 2019 and 2018, respectively. The income tax benefits recognized during the three months ended September 30, 2018 and nine months ended September 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 September 30, 2018 and nine months ended September 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	9 Months Ended
	Sep. 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 Nine Months Ended 2019 2018 2019 2018 Numerator: Net loss $ (21,227 ) $ (42,646 ) $ (82,257 ) $ (91,358 ) Accretion of redeemable convertible preferred stock to redemption value — — — (644 ) Net loss attributable to common stockholders $ (21,227 ) $ (42,646 ) $ (82,257 ) $ (92,002 ) Denominator: Weighted average common shares outstanding—basic and diluted 51,891,157 44,036,206 48,574,275 17,949,026 Net loss per share attributable to common stockholders—basic and diluted $ (0.41 ) $ (0.97 ) $ (1.69 ) $ (5.13 ) The Company excluded 99,468 shares and 333,473 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 September 30, 2019 and 2018, respectively, because those shares had not vested. The Company excluded 147,842 shares and 411,644 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 nine months ended September 30, 2019 and 2018, respectively, because those shares had not vested. The Company’s potentially dilutive securities, which include stock options and unvested restricted common 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: September 30, 2019 2018 Options to purchase common stock 8,568,506 6,241,584 Unvested restricted common stock 72,677 290,267 8,641,183 6,531,851

Leases

Leases	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Leases	12. Leases The Company is a lessee under two 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 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 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 nine the real estate lease for The components of lease cost were as follows (dollar amounts in thousands): Three Months Ended Nine Months Ended Lease cost Operating lease cost $ 673 $ 2,019 Short-term lease cost — — Total lease cost $ 673 $ 2,019 Other information Operating cash flows from operating leases $ 650 $ 1,933 Operating lease liabilities arising from obtaining right-of-use $ — $ — Weighted-average remaining lease term 8 8 Weighted-average discount rate 17.5 % 17.5 % During the three and nine months ended September 30, 2018, the company recorded rent expense of $0.7 million and $2.4 million, respectively. As of September 30, 2019, maturities of operating lease liabilities are as follows (in thousands): September 30, 2019 2019 $ 650 2020 2,659 2021 2,737 2022 2,818 2023 2,860 2024 and thereafter 13,097 Total future minimum lease payments 24,821 Less: imputed interest (12,102 ) Present value of lease liabilities $ 12,719 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. Suite Retention and Development Agreement In September 2019, the Company entered into a suite retention and development agreement with Albany Molecular Research, Inc. (“AMRI”) under build-out obtained. The build-out is expected to be completed by the end of the second quarter of 2020. The five build-out the Company has the right to extend for an additional three build-out (“Build-Out Build-Out Build-Out Build-Out , , , the build-out build-out The Company has determined this is a lease under ASC 842. As of September 30, 2019, the Company has determined that it does not have control of the space, as defined in ASC 842, during the build-out and as such, this lease was not included in the ROU asset or lease liabilities on the Company’s condensed consolidated balance sheet.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies Research, Supply and License Agreements 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 MIT Research Agreements In September 2019, the Company entered into a research agreement with the Massachusetts Institute of Technology (“MIT”) pursuant to which the Company is obligated to reimburse MIT up to $4.1 million for specified direct and indirect costs to be incurred from January 2020 through December 2022 for specified research activities conducted for the Company (the “2019 MIT Agreement”). As of September 30, 2019, the Company had not made any payments to MIT towards the total committed amount. There were no research and development expenses related to this agreement during the three and nine months ended September 30, 2019. As of September 30, 2019, there were no amounts payable by the Company under the agreement. The 2019 MIT Agreement expires in December 2022 and may be extended thereafter by mutual agreement of the parties. In November 2016, the Company entered into a research agreement with MIT pursuant to which the Company was 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 (the “2016 MIT Agreement”). As of September 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 September 30, 2019 and 2018, respectively, and $0.7 million and $0.9 million during the nine months ended September 30, 2019 and 2018, respectively. As of September 30, 2019, there were no amounts payable by the Company under the agreement. As amended, the 2016 MIT Agreement expired in October 2019. 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 non-coding 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 September 30, 2019 and 2018 and paid $0.2 million and $0.1 million during the nine months ended September 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 country-by-country last-to-expire MIT-licensed 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, i f Research and development expenses related to this agreement totaled less than $0.1 million during each of the three months ended September 30, 2019 and 2018, and $0.1 million and $ 0.1 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 September 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	9 Months Ended
	Sep. 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 months ended September 30, 2019 and 2018, the Company did not record any general and administrative expenses related to this agreement. During the nine months ended September 30, 2019 and 2018, the Company recorded general and administrative expenses of $0 and $0.1 million, respectively, related to this agreement. During the three and nine months ended September 30, 2019 and 2018, the Company paid Mr. Lynch $0, $0, $11,250 and $0.1 million, respectively, in connection with his services provided under the agreement. As of September 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 in May 2019, entities affiliated with Baupost Group, L.L.C. (“Baupost”), 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. Public Offering In connection with a public offering of the Company’s common stock in September 2019, Baupost purchased 5,000,000 shares of the Company’s common stock at a price per share of $10.00 for an aggregate purchase price of $50.0 million.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 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 Accounting Standards Codification (“ASC”) . Subsequent to the issuance of Topic , the FASB clarified the guidance through several ASUs; hereinafter the collection of lease guidance is referred to as “ASC ”. 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 – 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 - 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)	9 Months Ended
	Sep. 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 September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Collaboration revenue $ 1,266 $ 238 $ 3,914 $ 238
Balance of Contract Liabilities Related to Collaboration Agreements	The following table presents the balance of the Company’s contract liabilities (in thousands): September 30, December 31, Contract liabilities Deferred revenue $ 42,649 $ 44,413

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	9 Months Ended
	Sep. 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: September 30, 2019 Estimated Life Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 $ 45,992 $ (1,588 ) $ — $ 44,404 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 $ (1,588 ) $ (18,559 ) $ 86,695 December 31, 2018 Estimated Life Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 $ 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)	9 Months Ended
	Sep. 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 September 30, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 48,872 $ — $ 48,872 U.S. government agency bonds — 111,233 — 111,233 $ — $ 160,105 $ — $ 160,105 Liabilities: Contingent consideration $ — $ — $ 100,399 $ 100,399 $ — $ — $ 100,399 $ 100,399 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 September 30, 2019 and December 31, Fair Value at Projected Year of Payment September 30, December 31, Earnout payments 2026 - 2039 $ 93,078 $ 94,999 Milestone payments 2026 - 2030 7,321 8,643 $ 100,399 $ 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 Discontinuation of MRT5201 (23,174 ) Increase 19,931 Balance as of September 30, 2019 $ 100,399

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2019
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): September 30, December 31, Laboratory equipment $ 8,788 $ 7,012 Computer equipment 780 686 Office equipment 883 836 Leasehold improvements 5,634 5,635 Construction in progress 1,237 959 17,322 15,128 Less: Accumulated depreciation and amortization (6,642 ) (4,883 ) $ 10,680 $ 10,245

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2019
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): September 30, December 31, Accrued external research and development expenses $ 4,221 $ 1,901 Accrued employee compensation and benefits 3,283 2,933 Accrued consultant and professional fees 1,263 977 Other — 736 $ 8,767 $ 6,547

Incentive Stock Options and R_2

Incentive Stock Options and Restricted Stock (Tables)	9 Months Ended
	Sep. 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 2,723,200 $ 8.52 Exercised (232,207 ) $ 6.47 Forfeited (158,493 ) $ 8.02 Outstanding as of September 30, 2019 8,568,506 $ 8.04 8.67 $ 16,352 Exercisable as of September 30, 2019 3,071,842 $ 7.65 8.21 $ 6,970 Vested and expected to vest as of September 30, 2019 8,568,506 $ 8.04 8.67 $ 16,352 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 (1,783 ) $ 1.28 Vested restricted common stock (144,688 ) $ 1.27 Unvested restricted common stock outstanding as of September 30, 2019 72,677 $ 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 Nine Months Ended 2019 2018 2019 2018 Research and development expenses $ 1,301 $ 639 $ 3,454 $ 3,092 General and administrative expenses 1,598 796 4,954 2,601 $ 2,899 $ 1,435 $ 8,408 $ 5,693
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: Nine Months Ended September 30, 2019 2018 Risk-free interest rate 2.39 % 2.81 % Expected term (in years) 6.0 6.0 Expected volatility 73.1 % 75.6 % Expected dividend yield 0 % 0 %

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 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 Nine Months Ended 2019 2018 2019 2018 Numerator: Net loss $ (21,227 ) $ (42,646 ) $ (82,257 ) $ (91,358 ) Accretion of redeemable convertible preferred stock to redemption value — — — (644 ) Net loss attributable to common stockholders $ (21,227 ) $ (42,646 ) $ (82,257 ) $ (92,002 ) Denominator: Weighted average common shares outstanding—basic and diluted 51,891,157 44,036,206 48,574,275 17,949,026 Net loss per share attributable to common stockholders—basic and diluted $ (0.41 ) $ (0.97 ) $ (1.69 ) $ (5.13 )
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: September 30, 2019 2018 Options to purchase common stock 8,568,506 6,241,584 Unvested restricted common stock 72,677 290,267 8,641,183 6,531,851

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Schedule of Lease Cost	The components of lease cost were as follows (dollar amounts in thousands): Three Months Ended Nine Months Ended Lease cost Operating lease cost $ 673 $ 2,019 Short-term lease cost — — Total lease cost $ 673 $ 2,019 Other information Operating cash flows from operating leases $ 650 $ 1,933 Operating lease liabilities arising from obtaining right-of-use $ — $ — Weighted-average remaining lease term 8 8 Weighted-average discount rate 17.5 % 17.5 %
Schedule of maturities of operating lease liabilities	As of September 30, 2019, maturities of operating lease liabilities are as follows (in thousands): September 30, 2019 2019 $ 650 2020 2,659 2021 2,737 2022 2,818 2023 2,860 2024 and thereafter 13,097 Total future minimum lease payments 24,821 Less: imputed interest (12,102 ) Present value of lease liabilities $ 12,719
Schedule of Future Minimum Rental Commitments for Operating Leases	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

Sep. 30, 2019USD ($)shares

Jun. 30, 2019USD ($)

Mar. 31, 2019USD ($)

Sep. 30, 2018USD ($)

Jun. 30, 2018USD ($)

Mar. 31, 2018USD ($)

Sep. 30, 2019USD ($)Subsidiary

Sep. 30, 2018USD ($)

Dec. 31, 2018USD ($)Disease

Sep. 20, 2019$ / shares

Jul. 03, 2019USD ($)

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

Payments of other offering expenses

$ 4,089

Accumulated deficit

$ (328,460)

$ (328,460)

$ (246,203)

Net loss

(21,227)

$ (27,832)

$ (33,198)

$ (42,646)

$ (27,503)

$ (21,209)

(82,257)

$ (91,358)

Cash, cash equivalents and short-term investments

210,200

210,200

Sale from time to time of equity and debt securities

$ 250,000

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

Open Market Sale Agreement [Member]

Initial Public Offering [Line Items]

Market sale aggreement of common stock

$ 50,000

Public Offering [Member]

Initial Public Offering [Line Items]

Payments of other offering expenses

$ 600

Issuance of common, shares | shares

9,000,000

Gross proceeds from issuance of public offering

$ 90,000

Underwriting discounts and commissions

$ 5,400

Common stock issued and sold, per share | $ / shares

$ 10

IPO [Member]

Initial Public Offering [Line Items]

Issuance of common, shares | shares

9,714,371

Common stock issued and sold, per share | $ / shares

$ 13

Private Placement [Member]

Initial Public Offering [Line Items]

Gross proceeds from issuance of private placement

$ 47,500

Payments of other offering expenses

$ 600

Issuance of common, shares | shares

5,582,940

Common stock issued and sold, per share | $ / shares

$ 8.50

Placement agent fees

$ 2,800

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019
Indefinite-lived Intangible Assets [Line Items]
Right-of-use assets, net	$ 10,527
Operating lease, liability	$ 12,719
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) - Sanofi Agreement [Member] $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)Disease
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Short-term receivables		$ 0.7	$ 0.8
Number of infectious disease pathogens for vaccine development | Disease			5
Period of research term			3 years
Upfront payment received		$ 45
Reimbursable development costs payable period		60 days
Additional fee per added pathogen option			$ 5
Maximum development and regulatory milestone payment receivable			63
Technology and process transfer milestone payment receivable			10
Royalty payment term		10 years
Reduction in transaction price	$ 10
Non-refundable upfront payment	45
Estimated reimbursable employee cost	32.6
Estimated reimbursable development cost	54.5
Estimated milestone payments	$ 19
Revenue recognizing period		8 years
Revenue recognized from contract liabilities		$ 1.8
Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Extended period of research term		8 years
Receivable from collaboration			805
Sales milestone payment receivable			$ 85

Sanofi Collaboration and Lice_4

Sanofi Collaboration and License Agreement - Summary of Collaboration Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration revenue	$ 1,266	$ 238	$ 3,914	$ 238

Sanofi Collaboration and Lice_5

Sanofi Collaboration and License Agreement - Balance of Contract Liabilities Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Contract liabilities
Deferred revenue	$ 42,649	$ 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	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, accumulated amortization	$ (1,588)	$ (1,588)	$ (397)
Definite-lived intangible assets, Impairment Charge	(18,559)	(18,559)
Total intangible assets, gross carrying amount	106,842	106,842	106,842
Total intangible assets, net carrying amount	86,695	$ 86,695	$ 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	$ 45,992
Definite-lived intangible assets, accumulated amortization	(1,588)	(1,588)	(397)
Definite-lived intangible assets, net carrying amount	44,404	44,404	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	42,291
In-Process Research and Development [Member] | OTC Deficiency [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, Impairment Charge	(18,600)	(18,600)
Indefinite-lived intangible assets, gross carrying amount/net carrying amount	$ 18,559	$ 18,559	$ 18,559

Intangible Assets and Goodwil_3

Intangible Assets and Goodwill - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Finite Lived Intangible Assets [Line Items]
Impairment of Intangible Assets, Finite-lived	$ 0
indefinite lived intangible assets Impairment charge	18,559,000	$ 18,559,000
Accrued expenses	8,767,000	8,767,000	$ 6,547,000
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
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	400,000	1,200,000
OTC Deficiency [Member]
Finite Lived Intangible Assets [Line Items]
Accrued expenses	300,000	300,000
OTC Deficiency [Member] | Prepaid Expenses and Other Current Assets [Member]
Finite Lived Intangible Assets [Line Items]
short-term receivables	800,000	800,000
In Process Research and Development [Member] | OTC Deficiency [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
indefinite lived intangible assets Impairment charge	$ 18,600,000	$ 18,600,000

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	Sep. 30, 2019	Dec. 31, 2018
Assets:
Total, assets	$ 160,105	$ 112,222
Liabilities:
Total, liabilities	100,399	103,642
Fair Value, Inputs, Level 2
Assets:
Total, assets	160,105	112,222
Fair Value, Inputs, Level 3
Liabilities:
Total, liabilities	100,399	103,642
Money Market Funds
Assets:
Total, assets	48,872	23,318
Money Market Funds | Fair Value, Inputs, Level 2
Assets:
Total, assets	48,872	23,318
U.S. Government Agency Bonds
Assets:
Total, assets	111,233	88,904
U.S. Government Agency Bonds | Fair Value, Inputs, Level 2
Assets:
Total, assets	111,233	88,904
Contingent Consideration
Liabilities:
Total, liabilities	100,399	103,642
Contingent Consideration | Fair Value, Inputs, Level 3
Liabilities:
Total, liabilities	$ 100,399	$ 103,642

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Schedule of Unobservable Inputs and Fair Value Components of Contingent Consideration (Detail) - Contingent Consideration - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	$ 100,399	$ 103,642
Earnout Payments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	$ 93,078	$ 94,999
Earnout Payments | Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2026	2026
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	$ 7,321	$ 8,643
Milestone Payments | Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2026	2026
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_5

Fair Value of Financial Assets and Liabilities - Schedule of Total Acquisition Related Contingent Consideration Liability (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition, Contingent Consideration [Line Items]
Beginning Balance			$ 103,642
Discontinuation of MRT5201 and Increase in fair value of contingent consideration	$ (19,834)	$ 26,829	(3,243)	$ 39,589
Ending Balance	$ 100,399		100,399
Discontinuation of MRT5201
Business Acquisition, Contingent Consideration [Line Items]
Discontinuation of MRT5201 and Increase in fair value of contingent consideration			(23,174)
Contingent Consideration
Business Acquisition, Contingent Consideration [Line Items]
Discontinuation of MRT5201 and Increase in fair value of contingent consideration			$ 19,931

Fair Value of Financial Asset_6

Fair Value of Financial Assets and Liabilities - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 98,930,000	$ 38,864,000	$ 98,930,000	$ 38,864,000	$ 55,199,000
Short-term investments amortized cost	110,500,000		110,500,000
Short-term investments unrealized gain			700,000
Short-term investments fair value	111,200,000		111,200,000
Increase decrease in contingent consideration liability	$ (19,834,000)	$ 26,829,000	$ (3,243,000)	$ 39,589,000
Contingent Consideration
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Discount Rate	13.5		13.5		14.5
Increase decrease in contingent consideration liability			$ 19,931,000
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	$ 48,900,000		$ 48,900,000		$ 23,300,000
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value, assets transfers into (out of) Level 3			0		0
Fair value,assets transfers from Level 2 to Level 1	0		0		0
Fair value,assets transfers from Level 1 to Level 2	0		0		0
Fair value, liabilities transfers from Level 1 to Level 2	0		0		0
Fair value, liabilities transfers from Level 2 to Level 1	$ 0		0		0
Fair Value, liabilities transfers into (out of) Level 3			0		$ 0
In Process Research and Development [Member] | OTC Deficiency [Member] | Shires Mrt Program [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase decrease in contingent consideration liability			$ 23,200,000

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 17,322	$ 15,128
Less: Accumulated depreciation and amortization	(6,642)	(4,883)
Property and equipment, net	10,680	10,245
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	8,788	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	$ 1,237	$ 959

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 0.6	$ 0.5	$ 1.8	$ 1.9

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 4,221	$ 1,901
Accrued employee compensation and benefits	3,283	2,933
Accrued consultant and professional fees	1,263	977
Other		736
Total accrued expenses	$ 8,767	$ 6,547

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock - Additional Information (Detail) - shares	Sep. 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	Sep. 30, 2019	Sep. 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					60,020,725		45,139,955
Unrecognized compensation cost related to unvested stock-based awards					$ 26,700
Unrecognized compensation cost, period for recognition					2 years 6 months 25 days
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Intrinsic value of stock options, exercised					$ 600	$ 200
Weighted average grant-date fair value					$ 5.58	$ 5.97
2018 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares reserved for issuance					4,820,561
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
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					142,262
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

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	9 Months Ended	12 Months Ended
	Sep. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares
Number of shares
Number of shares, beginning balance | shares	6,236,006
Number of shares, granted | shares	2,723,200
Number of shares, exercised | shares	(232,207)
Number of shares, forfeited | shares	(158,493)
Number of shares, ending balance | shares	8,568,506	6,236,006
Number of shares, exercisable | shares	3,071,842	1,827,004
Number of shares, vested and expected to vest, ending balance | shares	8,568,506	6,236,006
Weighted average exercise price
Weighted average exercise price, beginning balance | $ / shares	$ 7.78
Weighted average exercise price, granted | $ / shares	8.52
Weighted average exercise price, exercised | $ / shares	6.47
Weighted average exercise price, forfeited | $ / shares	8.02
Weighted average exercise price, ending balance | $ / shares	8.04	$ 7.78
Weighted average exercise price, exercisable | $ / shares	7.65	7.16
Weighted average exercise price, vested and expected to vest, ending balance | $ / shares	$ 8.04	$ 7.78
Weighted average remaining contractual term
Weighted average remaining contractual term, ending balance	8 years 8 months 1 day	8 years 8 months 26 days
Weighted average remaining contractual term, exercisable	8 years 2 months 15 days	7 years 11 months 12 days
Weighted average remaining contractual term, vested and expected to vest, ending balance	8 years 8 months 1 day	8 years 8 months 26 days
Intrinsic value
Intrinsic value, ending balance | $	$ 16,352	$ 1,104
Intrinsic value, exercisable | $	6,970	632
Intrinsic value, vested and expected to vest, ending balance | $	$ 16,352	$ 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]	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.39%	2.81%
Expected term (in years)	6 years	6 years
Expected volatility	73.10%	75.60%
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]	9 Months Ended
	Sep. 30, 2019$ / sharesshares
Number of shares
Number of shares, beginning balance | shares	219,148
Number of shares, forfeited | shares	(1,783)
Number of shares, vested | shares	(144,688)
Number of shares, ending balance | shares	72,677
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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Stock-based compensation expense	$ 2,899	$ 1,435	$ 8,408	$ 5,693
Research and Development Expenses [Member]
Stock-based compensation expense	1,301	639	3,454	3,092
General and Administrative Expenses [Member]
Stock-based compensation expense	$ 1,598	$ 796	$ 4,954	$ 2,601

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Benefit from income taxes	$ 0	$ 2,524	$ 486	$ 5,126

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						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss	$ (21,227)	$ (27,832)	$ (33,198)	$ (42,646)	$ (27,503)	$ (21,209)	$ (82,257)	$ (91,358)
Accretion of redeemable convertible preferred stock to redemption value								(644)
Net loss attributable to common stockholders	$ (21,227)			$ (42,646)			$ (82,257)	$ (92,002)
Denominator:
Weighted average common shares outstanding—basic and diluted	51,891,157			44,036,206			48,574,275	17,949,026
Net loss per share attributable to common stockholders—basic and diluted	$ (0.41)			$ (0.97)			$ (1.69)	$ (5.13)

Net Loss per Share - Additional

Net Loss per Share - Additional Information (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 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			8,641,183	6,531,851
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			72,677	290,267
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	99,468	333,473	147,842	411,644

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	9 Months Ended
	Sep. 30, 2019	Sep. 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	8,641,183	6,531,851
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	8,568,506	6,241,584
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	72,677	290,267

Leases - Additional Information

Leases - Additional Information (Details) $ in Millions	1 Months Ended			3 Months Ended	9 Months Ended
	Sep. 30, 2019USD ($)	Mar. 31, 2018	Jun. 30, 2017USD ($)ft²	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)Lease	Sep. 30, 2018USD ($)
Lessee Lease Description [Line Items]
Number of operating leases | Lease					2
Remaining term of lease	8 years				8 years
Rent expense				$ 0.7		$ 2.4
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
Percentage of annual increase in operating lease			3.00%
Cash deposit collateral			$ 1
Operating Lease Term Of Contract		5 years	10 years
Suite Retention And Development Agreement [Member]
Lessee Lease Description [Line Items]
Commitment to Build Out Cost	$ 6				$ 6
Shared Overage cost commitment	$ 11				$ 11
Maximum [Member] | Lexington, Massachusetts [Member]
Lessee Lease Description [Line Items]
Remaining term of lease	9 years				9 years
Albany Molecular Research, Inc. ("AMRI") [Member] | Suite Retention And Development Agreement [Member]
Lessee Lease Description [Line Items]
Lease, term of contract	5 years				5 years
Monthly lease payments	$ 1
Payment terms of Build-Out Costs	In the event the Build-Out Costs exceed $6.0 million, the Company and AMRI will share overage costs equally, up to $11.0 million.
Operating Lease Term Of Contract	5 years				5 years
Operating Lease Renewal Term	3 years				3 years
Operating lease option to extend description	the Company has the right to extend for an additional three years
Percentage Of Increase In The Monthly Rental Expense	3.00%
Albany Molecular Research, Inc. ("AMRI") [Member] | Suite Retention And Development Agreement [Member] | Other Noncurrent Assets [Member]
Lessee Lease Description [Line Items]
Build Out Costs paid	$ 2.5
Albany Molecular Research, Inc. ("AMRI") [Member] | Suite Retention And Development Agreement [Member] | Prepaid Expenses and Other Current Assets [Member]
Lessee Lease Description [Line Items]
Payments For Purchase Of Deliverables	$ 1

Leases - Schedule of Lease Cost

Leases - Schedule of Lease Cost (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019USD ($)	Sep. 30, 2019USD ($)
Lease Cost
Operating lease cost	$ 673	$ 2,019
Total lease cost	673	2,019
Operating cash flows from operating leases	$ 650	$ 1,933
Weighted-average remaining lease term	8 years	8 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	Sep. 30, 2019	Dec. 31, 2018
Leases [Abstract]
2019	$ 650	$ 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	13,097	13,096
Total future minimum lease payments	24,821	$ 26,549
Less: imputed interest	(12,102)
Present value of lease liabilities	$ 12,719

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Nov. 30, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Loss Contingencies [Line Items]
Research and development			$ 17,295,000	$ 12,933,000	$ 51,343,000	$ 40,854,000
Research agreement, expiration period					2022-12
Research agreement, payable amount	$ 4,221,000		4,221,000		$ 4,221,000		$ 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					$ 18,600,000
Purchase commitments, year 2019	4,200,000		4,200,000		4,200,000
Purchase commitments, year 2020	500,000		500,000		500,000
Purchase commitments, year 2021	3,500,000		3,500,000		3,500,000
Purchase commitments, year 2022	3,500,000		3,500,000		3,500,000
Purchase commitments, year 2023	3,500,000		3,500,000		3,500,000
Purchase commitments, year 2024	3,500,000		3,500,000		3,500,000
Research and development			1,800,000	800,000	5,400,000	2,500,000
MIT Research Agreement [Member]
Loss Contingencies [Line Items]
Research and development			200,000	300,000	$ 700,000	900,000
Research agreement, committed amount	4,100,000	$ 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	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		$ 0
Annual license maintenance payments	200,000		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		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 ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2012	Sep. 20, 2019	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	$ 0	$ 0	$ 100
Amount paid for services		0	$ 11,250	0	$ 100,000
Amount due to related party	$ 0	$ 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		$ 8.50
Sale of Stock, Consideration Received on Transaction				$ 20,000,000
Public Offering [Member]
Related Party Transaction [Line Items]
Sale of Stock, Price Per Share							$ 10
Public Offering [Member] | Baupost Group LLC [Member]
Related Party Transaction [Line Items]
Sale of Stock, Number of Shares Issued in Transaction	5,000,000
Sale of Stock, Price Per Share	$ 10	$ 10		$ 10
Sale of Stock, Consideration Received on Transaction	$ 50,000,000