Telesis Bio 10-Q7 May 202020 Q1 Quarterly reportFinancial data

Company Profile

Cover Page

Cover Page - shares	3 Months Ended
Cover Page - shares	Mar. 31, 2020	May 05, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
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	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Address, State or Province	MA
Title of 12(b) Security	Common Stock
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		62,908,826

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 97,468	$ 84,580
Short-term investments	57,627	104,098
Collaboration receivables	6,228	4,596
Prepaid expenses and other current assets	6,921	9,391
Restricted cash	950	950
Total current assets	169,194	203,615
Property and equipment, net	13,781	12,539
Right-of-use assets, net	10,268	10,400
Goodwill	21,359	21,359
Intangible assets, net	84,844	85,536
Other assets	5,796	2,752
Total assets	305,242	336,201
Current liabilities:
Accounts payable	6,297	15,968
Accrued expenses	7,478	7,072
Current portion of deferred revenue	26,760	18,100
Current portion of operating lease liability	573	530
Total current liabilities	41,108	41,670
Contingent consideration	94,203	103,655
Deferred revenue, net of current portion	15,328	25,256
Operating lease liability, net of current portion	11,931	12,084
Total liabilities	162,570	182,665
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized as of March 31, 2020 and December 31, 2019, respectively; no shares issued and outstanding as of March 31, 2020 and December 31, 2019
Common stock, $0.001 par value; 200,000,000 shares authorized as of March 31, 2020 and December 31, 2019; 60,037,663 shares and 60,022,067 shares issued and outstanding as of March 31, 2020 and December 31, 2019, respectively	60	60
Additional paid-in capital	515,535	512,231
Accumulated deficit	(373,778)	(359,496)
Accumulated other comprehensive income	855	741
Total stockholders' equity	142,672	153,536
Total liabilities and stockholders' equity	$ 305,242	$ 336,201

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
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	60,037,663	60,022,067
Common stock, shares outstanding	60,037,663	60,022,067

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Collaboration revenue	$ 4,654	$ 1,474
Operating expenses:
Research and development	21,439	17,423
General and administrative	7,458	6,554
Change in fair value of contingent consideration	(9,452)	11,702
Total operating expenses	19,445	35,679
Loss from operations	(14,791)	(34,205)
Interest income, net	509	521
Loss before benefit from income taxes	(14,282)	(33,684)
Benefit from income taxes	0	486
Net loss	$ (14,282)	$ (33,198)
Net loss per share—basic and diluted	$ (0.24)	$ (0.74)
Weighted average common shares outstanding—basic and diluted	60,008,217	45,004,521

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net loss	$ (14,282)	$ (33,198)
Other comprehensive income (loss):
Unrealized gains on available-for-sale securities, net of tax of $0	114	155
Comprehensive loss	$ (14,168)	$ (33,043)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Unrealized gains on available-for-sale securities, net of tax	$ 0	$ 0

Condensed Consolidated Statem_4

Condensed Consolidated Statements Of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income [Member]
Beginning balances at Dec. 31, 2018	$ 125,295	$ 45	$ 371,257	$ (246,203)	$ 196
Beginning balance, Shares at Dec. 31, 2018		45,139,955
Exercise of stock options	897		897
Exercise of stock options, Shares		154,484
Stock-based compensation expense	1,959		1,959
Unrealized gains on available-for-sale securities	155				155
Net loss	(33,198)			(33,198)
Ending balances at Mar. 31, 2019	95,108	$ 45	374,113	(279,401)	351
Ending balance, shares at Mar. 31, 2019		45,294,439
Beginning balances at Dec. 31, 2019	153,536	$ 60	512,231	(359,496)	741
Beginning balance, Shares at Dec. 31, 2019		60,022,067
Exercise of stock options	132		132
Exercise of stock options, Shares		15,596
Stock-based compensation expense	3,172		3,172
Unrealized gains on available-for-sale securities	114				114
Net loss	(14,282)			(14,282)
Ending balances at Mar. 31, 2020	$ 142,672	$ 60	$ 515,535	$ (373,778)	$ 855
Ending balance, shares at Mar. 31, 2020		60,037,663

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (14,282)	$ (33,198)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	1,351	1,025
Stock-based compensation expense	3,172	1,959
Change in fair value of contingent consideration	(9,452)	11,702
Deferred income tax benefit		(486)
Changes in operating assets and liabilities:
Collaboration receivables	(1,633)	(482)
Prepaid expenses and other assets	(397)	(285)
Right-of-use assets	132	115
Accounts payable	(9,545)	(1,048)
Accrued expenses	584	(71)
Lease liability	(110)	(74)
Deferred revenue	(1,267)	(688)
Net cash used in operating activities	(31,447)	(21,531)
Cash flows from investing activities:
Purchases of investments	(27,409)
Sales and maturities of investments	73,994	28,905
Purchases of property and equipment	(2,325)	(1,039)
Net cash provided by investing activities	44,260	27,866
Cash flows from financing activities:
Payments of public offering costs	(57)
Proceeds from option exercises	132	897
Net cash provided by financing activities	75	897
Net increase in cash, cash equivalents and restricted cash	12,888	7,232
Cash, cash equivalents and restricted cash at beginning of period	85,530	56,224
Cash, cash equivalents and restricted cash at end of period	98,418	63,456
Cash, cash equivalents and restricted cash at end of period:
Cash and cash equivalents	97,468	62,431
Restricted cash	950	1,025
Total cash, cash equivalents and restricted cash at end of period	98,418	63,456
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses	764	$ 95
Deferred offering costs included in accounts payable and accrued expenses	$ 120

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	3 Months Ended
Nature of the Business and Basis of Presentation	Mar. 31, 2020
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 designed to be 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 its MRT platform to treat pulmonary diseases caused by insufficient protein production or where production of proteins can modify disease. The Company’s MRT platform may also be applied to produce various classes of treatments, such as therapeutic antibodies or vaccines in areas such as infectious disease and oncology, and it has an ongoing mRNA vaccine development program. The outbreak of a novel strain of virus named SARS-CoV-2 (“COVID-19”) COVID-19 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 COVID-19 The Company is leveraging its lung delivery platform and focusing its preclinical research efforts on identifying lead product candidates for a next-generation CF program, as well as beyond CF in additional pulmonary diseases with unmet medical need, including primary ciliary dyskinesia, idiopathic pulmonary fibrosis and pulmonary arterial hypertension. The Company has a collaboration with Sanofi Pasteur, Inc. (“Sanofi”), the vaccines global business unit of Sanofi S.A., to develop infectious disease vaccines using the Company’s mRNA technology for up to five infectious disease pathogens. Three of the target pathogens have been identified, of which one is influenza, the second is another viral pathogen and the third is a bacterial pathogen. On March 26, 2020, the Company and Sanofi amended this collaboration to include the development of mRNA vaccines against SARS-CoV-2 COVID-19 in vivo In September 2019, the Company announced its decision to discontinue the development of MRT5201, a liver targeted treatment for ornithine transcarbamylase (“OTC”) deficiency (see Notes 4 and 5). The Company’s decision to discontinue the development of MRT5201 for OTC deficiency was based on data from preclinical studies completed in 2019, the results of 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 preparation of the accompanying condensed consolidated financial statements requires the Company to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, equity, revenues and expenses and related disclosure of contingent assets and liabilities. On an ongoing basis the Company evaluates its estimates, judgments and methodologies. The Company bases its estimates on historical experience and on various other assumptions that it believes are reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenues and expenses. The full extent to which the COVID-19 COVID-19 COVID-19, COVID-19 The accompanying condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its two wholly owned subsidiaries, Translate Bio MA, Inc. and Translate Bio Securities Corporation, from their date of incorporation. All intercompany accounts and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated balance sheet as of March 31, 2020, the unaudited condensed consolidated statements of operations and of comprehensive loss for the three months ended March 31, 2020 and 2019, the unaudited condensed consolidated statements of stockholders’ equity for the three months ended March 31, 2020 and 2019 and the unaudited condensed consolidated statements of cash flows for the three months ended March 31, 2020 and 2019 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. The accompanying balance sheet as of December 31, 2019 has been derived from the Company’s audited financial statements for the year ended December 31, 2019. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. The accompanying unaudited interim condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K 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, reflects all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of March 31, 2020, the results of its operations for the three months ended March 31, 2020 and 2019, and its cash flows for the three months ended March 31, 2020 and 2019. The financial data and other information disclosed in these notes related to the three months ended March 31, 2020 and 2019 are also unaudited. The results for the three months ended March 31, 2020 are not necessarily indicative of results to be expected for the year ending December 31, 2020, any other interim periods, or any future year or period. Sales of Common Stock In July 2019, the Company filed a universal shelf registration statement on Form S-3 No. 333-232543). In July 2019, the Company entered into an Open Market Sale Agreement SM million. The offer and sales of shares under the Sales Agreement were also registered under the Securities Act of 1933, as amended (the “Securities Act”), pursuant to the 2019 Shelf. Between April 1, 2020 and May 5, 2020, the Company settled transactions that occurred pursuant to the Sales Agreement, whereby the Company issued and sold an aggregate of 2,863,163 shares of its common stock between March 30, 2020 and May 1, 2020, resulting in gross proceeds of $37.9 million, before deducting commissions of $1.1 million and other offering expenses of $0.2 million (see Note 14). Sales of common stock through Jefferies may be made by any method that is deemed an “at the market” offering as defined in Rule 415 promulgated under the Securities Act. Jefferies has agreed to use its commercially reasonable efforts consistent with its normal trading and sales practices to sell the common stock based upon the Company’s instructions. The Company is not obligated to make any sales of its common stock under the Sales Agreement. On March 13, 2020, the Company filed a universal shelf registration statement on Form S-3 No. 333-237159). On March 13, 2020, the Company entered into Amendment No. 1 to the Open Market Sale Agreement SM dol l As of May 1 issued and million under the Sales Agreement. In the future, $62.1 million of shares of common stock remain available to be sold pursuant to the Sales Agreement, which sales, if any, would be made under the 2020 Shelf. Any shares sold subsequent to the effectiveness of the 2020 Shelf would be pursuant to the 2020 Shelf. Sanofi Pasteur Collaboration and Licensing Agreement In 2018, the Company entered into a collaboration and license agreement with Sanofi (the “Original Sanofi Agreement”) to SARS-CoV-2 Pursuant to the Sanofi Amendment, the Company and Sanofi agreed that no upfront fee is payable by Sanofi to the Company with respect to the addition of SARS-CoV-2 SARS-CoV-2 SARS-CoV-2 is a declared pandemic. The Company and Sanofi agreed to negotiate in good faith the royalty terms applicable to such products, which terms shall reflect the economic conditions applicable to commercializing such products and shall not exceed the royalty terms for the existing Licensed Fields. The Company and Sanofi have also agreed to jointly conduct early stage formulation and preclinical activities on a SARS-CoV-2 vaccine. The Company and Sanofi agreed to negotiate in good faith the terms under which further activities under this program will occur. 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 March 31, 2020, 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 Company’s initial public offering (the “IPO”) the upfront payment received under the Original 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 $14.3 million and $33.2 million for the three months ended March 31, 2020 and 2019, respectively. In addition, the Company had an accumulated deficit of $373.8 million as of March 31, 2020. The Company expects to continue to generate operating losses for the foreseeable future. As of May 7, 2020, the date of issuance of these unaudited interim condensed consolidated financial statements, the Company expects that its cash, cash equivalents and short-term investments of $155.1 million as of March 31, 2020, together with the net proceeds of approximately $36.6 million from issuances and sales of common stock pursuant to the Sales Agreement between March 30, 2020 and May 1, 2020, which transactions were settled between April 1, 2020 and May 5, 2020, will be sufficient to fund its operating expenses and capital expenditure requirements into the third quarter of 2021. The future viability of the Company beyond that point is dependent on the Company’s ability to raise additional capital to finance its operations. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all. The Company expects that its expenses will increase in connection with its ongoing business activities. As a result, the Company will need substantial additional funding to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through the sale of equity, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may be unable to raise additional funds or enter into such other agreements or arrangements when needed on favorable terms, or at all. If the Company is unable to obtain funding, the Company will be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. Based on its recurring losses and cash outflows from operations since inception, expectation of continuing operating losses and cash outflows from operations for the foreseeable future and the need to raise additional capital to finance its future operations, the Company concluded that there is substantial doubt about its ability to continue as a going concern.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
Summary of Significant Accounting Policies	Mar. 31, 2020
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, 2019, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. Recently Adopted Accounting Pronouncements In January 2017, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2017-04, Intangibles—Goodwill and Other: Simplifying the Test for Goodwill Impairment (Topic 350) In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606. Revenue from Contracts with Customers Collaborative Arrangements Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments available-for-sale In December 2019, the FASB issued ASU No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes step-up

Collaboration Agreement

Collaboration Agreement	3 Months Ended
Collaboration Agreement	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Sanofi Collaboration and License Agreement	3. Collaboration Agreement Sanofi Collaboration and License Agreement In 2018, the Company entered into the Original Sanofi Agreement to develop mRNA vaccines and an SARS-CoV-2 Pursuant to the Amended 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. Other than the Licensed Field of SARS-CoV-2, the non-clinical joint steering committee revised the collaboration timeline and the Company now estimates the completion of the transfer of manufacturing capabilities to be six years, or 2024. Pursuant to the Amended Sanofi Agreement, the Company and Sanofi have agreed to a governance structure, including committees and working groups, to manage the activities under the collaboration. If the Company and Sanofi do not mutually agree on certain decisions, Sanofi would be able to break a deadlock without the Company’s consent. The collaboration includes an estimated budget. Sanofi is responsible for paying reimbursable development costs, including the Company’s employee costs, out-of-pocket costs paid to third parties and manufacturing costs, up to a specified amount for the Licensed Field other than SARS-CoV-2, for which each party pays its own employee costs and share equally in the collective out of pocket costs. Under the terms of the Amended Sanofi Agreement, the Company has granted to Sanofi exclusive, worldwide licenses under applicable patents, patent applications, know-how , subject to certain limitations on the pathogens. If, prior to the exercise of the options by Sanofi, the Company receives a bona fide third-party offer to acquire rights to the field to which an option relates, the Company must notify Sanofi of such offer, and if Sanofi does not exercise its option as to the applicable field, such field will no longer be subject to the option. The Company and Sanofi retain the rights to perform their respective obligations and exercise their respective rights under the Amended Sanofi Agreement other than for the Licensed Field of SARS-CoV-2, preclinical non-exclusive, Sanofi has sole responsibility for all commercialization activities for mRNA vaccines in the Licensed Fields, other than SARS-CoV-2, non-clinical The Amended Sanofi Agreement provides that the Company is eligible to receive aggregate potential payments of up to $805.0 million from Sanofi, which includes the $45.0 million upfront payment the Company received in 2018, potential milestone payments and potential option exercise payments. Sanofi will also pay the Company $5.0 million with respect to each additional Licensed Field for which it exercises an option. Sanofi did not pay an upfront fee to the Company with respect to the addition of SARS-CoV-2 as a Licensed Field. 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. Notwithstanding the foregoing, milestone payments provisions of the Original Sanofi Agreement shall not apply to vaccine products for the prevention, treatment or cure of SARS-CoV-2 that are purchased by a governmental authority while SARS-CoV-2 is a declared pandemic. Pursuant to the Amended Sanofi Agreement, 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 Company and Sanofi agreed to negotiate in good faith the royalty terms applicable to such products. 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 may not fall below the Company’s royalty obligations to third parties plus a royalty of a low single-digit percentage. Royalty payments are payable on a product-by-product and country-by-country basis beginning on the launch of the product in the country until the later of the expiration of the last valid claim covering such product or years after the launch of such product in such country. If the Company commercializes any product covered by such a Sanofi patent right, the Company would pay Sanofi a royalty of a low single-digit percentage. Notwithstanding the foregoing, pursuant to the Amended Sanofi Agreement, royalty payments shall not apply to vaccine products for the prevention, treatment or cure of SARS-CoV-2 that are purchased by a governmental authority while SARS-CoV-2 is a declared pandemic, and instead the parties will negotiate in good faith royalty terms to apply to such vaccine products, which royalty terms shall reflect the economic conditions applicable to commercializing such vaccine products, and in no event will be higher than those set out in the Original Sanofi Agreement. Under the Sanofi Amendment, the Company and Sanofi agreed to negotiate in good faith an additional amendment related to activities directed to clinical development or commercialization of a product related to SARS-CoV-2, The Amended Sanofi Agreement provides that it will remain in effect until terminated in accordance with its terms. Either the Company or Sanofi may terminate the Amended Sanofi Agreement in its entirety if the other party is subject to certain insolvency proceedings. Either party may terminate the Amended Sanofi Agreement in its entirety or with respect to a particular Licensed Field, country or product if the other party materially breaches the Amended Sanofi Agreement and the breach remains uncured for a specified period, which may be extended in certain circumstances. Sanofi may also terminate the Amended 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 Amended Sanofi Agreement, Sanofi may continue to manufacture and commercialize the terminated products for a specified period of time, subject to Sanofi’s payment obligations. Moreover, in the event that the Company and Sanofi are unable to mutually agree on terms relating to the conduct of clinical development and commercialization of a product related to SARS-CoV-2, the Company has the right to terminate and revoke the license granted to Sanofi with respect to SARS-CoV-2 with SARS- CoV-2 shall cease to be a Licensed Field. Upon any such termination and revocation by the Company, the Company and Sanofi have agreed to negotiate in good faith a termination agreement with respect to the Company’s use of any technology arising from the collaboration that is owned by Sanofi or jointly owned by the Company and Sanofi, that is necessary or useful to the further development or commercialization of a product directed to SARS-CoV-2. Accounting for the Amended Sanofi Agreement The Company accounts for the Sanofi Amendment entered into on March 26, 2020, with the exception of the Licensed Field for SARS-CoV-2, The Company identified the following promised goods or services contained in the Original 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 re-evaluated re-evaluate joint steering committee agreed non-refundable out-of-pocket 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 2020 2019 Collaboration revenue $ 4,654 $ 1,474 The following table presents the balance of the Company’s contract liabilities (in thousands): March 31, December 31, Contract liabilities Deferred revenue $ 42,088 $ 43,356 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 six years. Revenue recognized from contract liabilities was $1.3 million and $0.7 million during the three months ended March 31, 2020 and 2019, respectively. The Company evaluated the Sanofi Amendment, with respect to the Licensed Field for SARS-COV-2, under the ASC 606 contract modification guidance and determined that the Sanofi Amendment, with respect to the Licensed Field for SARS-COV-2, should be accounted for as a separate contract as it added additional distinct goods and services at a stand-alone price. The Original Sanofi Agreement will continue to be accounted for under ASC 606. The Company determined the Sanofi Amendment, with respect to the Licensed Field for SARS-COV-2, will be accounted for under ASC 808 as both parties are active participants and each party pays its own employee costs and share equally in the collective out of pocket costs. The Company will record payments to Sanofi under the amendment for SARS-COV-2 as research and development expense and reimbursements from Sanofi, if any, will be recorded as an offset to research and development expense. As of March 31, 2020, activities had not yet commenced with respect to the development of a vaccine against SARS-Cov-2

Intangible Assets and Goodwill

Intangible Assets and Goodwill	3 Months Ended
Intangible Assets and Goodwill	Mar. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	4. Intangible Assets and Goodwill 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. 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: March 31, 2020 Estimated Life Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 6 years $ 45,992 $ (3,439 ) $ — $ 42,553 Indefinite-lived intangible assets: IPR&D—CF Indefinite 42,291 — — 42,291 Total intangible assets, net $ 88,283 $ (3,439 ) $ — $ 84,844 December 31, 2019 Estimated Life Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 years $ 45,992 $ (2,747 ) $ — $ 43,245 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 $ (2,747 ) $ (18,559 ) $ 85,536 Identifiable intangible assets acquired in the acquisition of Shire’s MRT Program consisted of in-process Upon commencement of the Original 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 the intangible asset’s estimated life based on an economic consumption model. For the three months ended March 31, 2020 and 2019, the Company recorded amortization expense of $0.7 million and $0.5 million, respectively, related to the definite-lived MRT intangible asset. The estimated aggregate amortization expense for each of the five succeeding fiscal years is $8.2 million, $9.2 million, $9.3 million, $12.1 million and $4.5 million for the years ending December 31, 2020, 2021, 2022, 2023 and 2024, 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 1 st 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 three months ended March 31, 2020. 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	3 Months Ended
Fair Value of Financial Assets and Liabilities	Mar. 31, 2020
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 Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 53,850 $ — $ 53,850 U.S. government agency bonds — 57,627 — 57,627 $ — $ 111,477 $ — $ 111,477 Liabilities: Contingent consideration $ — $ — $ 94,203 $ 94,203 $ — $ — $ 94,203 $ 94,203 Fair Value Measurements as of December 31, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 56,591 $ — $ 56,591 U.S. government agency bonds — 104,098 — 104,098 $ — $ 160,689 $ — $ 160,689 Liabilities: Contingent consideration $ — $ — $ 103,655 $ 103,655 $ — $ — $ 103,655 $ 103,655 During the three months ended March 31, 2020 and the year ended December 31, 2019, there were no transfers between Level 1, Level 2 and Level 3. Cash equivalents as of March 31, 2020 and December 31, 2019 consisted of money market funds totaling $53.8 million and $56.6 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 March 31, 2020 and December 31, 2019 consisted of U.S. government agency bonds and were classified as available-for-sale Valuation of Contingent Consideration The contingent consideration liability related to the acquisition of Shire’s MRT Program in 2016 was classified as a 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 Fair Value at Projected Year of Payment March 31, 2020 December 31, 2019 Earnout payments 2026 - 2039 87,054 $ 96,097 Milestone payments 2026 - 2030 7,149 7,558 $ 94,203 $ 103,655 The discount rate used in the third-party valuation was 14.5% and 13.5% as of March 31, 2020 and December 31, 2019, 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, 2019 $ 103,655 Decrease in fair value of contingent consideration (9,452 ) Balance as of March 31, 2020 $ 94,203 The decrease in the fair value of contingent consideration was primarily due to an increase in the discount rate, partially offset by an increase in the fair value due to the continued progress of the MRT5005 program and the time value of money due to the passage of time.

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
Property and Equipment, Net	Mar. 31, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	6. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): March 31, December 31, Laboratory equipment $ 9,700 $ 9,044 Computer equipment 893 779 Office equipment 883 883 Leasehold improvements 5,635 5,635 Construction in progress 4,590 3,460 21,701 19,801 Less: Accumulated depreciation and amortization (7,920 ) (7,262 ) $ 13,781 $ 12,539 Depreciation and amortization expense related to property and equipment was $0.7 million and $0.6 million for the three months ended March 31, 2020 and 2019, respectively.

Accrued Expenses

Accrued Expenses	3 Months Ended
Accrued Expenses	Mar. 31, 2020
Payables and Accruals [Abstract]
Accrued Expenses	7. Accrued Expenses Accrued expenses consisted of the following (in thousands): March 31, December 31, Accrued external research and development expenses $ 3,879 $ 1,763 Accrued employee compensation and benefits 2,298 3,547 Accrued consultant and professional fees 1,174 1,390 Other 127 372 $ 7,478 $ 7,072

Incentive Stock Options and Res

Incentive Stock Options and Restricted Stock	3 Months Ended
Incentive Stock Options and Restricted Stock	Mar. 31, 2020
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Incentive Stock Options and Restricted Stock	8. Incentive Stock Options and Restricted Stock 2018 Equity Incentive Plan On March 7, 2018, the Company’s Board of Directors (the “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 Board of Directors. As of December 31, 2019, there were 4,829,847 shares of common stock reserved for issuance under the 2018 Plan. On January 1, 2020, the number of shares of common stock that may be issued under the 2018 Plan increased by 2,400,829 shares of common stock. During the three months ended March 31, 2020, a total of 12,723 shares issued under the 2016 Plan have been cancelled and rolled over to the 2018 Plan, such that 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 Board of Directors, subject to stockholder approval, adopted, and on June 15, 2018, the Company’s 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 Board of Directors. In December 2019, the Board of Directors elected to add no shares of common stock to the 2018 ESPP. As of March 31, 2020, 870,096 shares of common stock were reserved for issuance under this plan and no shares had been issued under the 2018 ESPP. 2016 Stock Incentive Plan The 2016 Plan provided 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, 2019 (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, 2019 8,646,378 $ 8.06 8.42 $ 3,687 Granted 2,369,168 $ 7.66 Exercised (15,596 ) $ 8.49 Forfeited (16,723 ) $ 9.16 Outstanding as of March 31, 2020 10,983,227 $ 7.97 8.56 $ 22,260 Exercisable as of March 31, 2020 4,287,057 $ 7.72 7.88 $ 9,685 Vested and expected to vest as of March 31, 2020 10,983,227 $ 7.97 8.56 $ 22,260 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 three months ended March 31, 2020 and 2019 was less than $0.1 million and $0.4 million, respectively. The weighted average grant-date fair value per share of stock options granted was $4.67 and $5.25 during the three months ended March 31, 2020 and 2019, respectively. Stock Option Valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company completed its IPO in July 2018 and therefore lacks company-specific historical and implied volatility information before that date. 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: Three Months Ended March 31, 2020 2019 Risk-free interest rate 0.86 % 2.47 % Expected term (in years) 6.1 6.0 Expected volatility 68.0 % 73.9 % Expected dividend yield 0 % 0 % Restricted Common Stock The following table summarizes the Company’s restricted stock activity since December 31, 2019: Number of Shares Weighted Average Grant-Date Fair Value Unvested restricted common stock outstanding as of December 31, 2019 34,168 $ 1.28 Forfeited restricted common stock — $ — Vested restricted common stock (27,405 ) $ 1.28 Unvested restricted common stock outstanding as of March 31, 2020 6,763 $ 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 March 31, 2020 2019 Research and development expenses $ 1,454 $ 869 General and administrative expenses 1,718 1,090 $ 3,172 $ 1,959 As of March 31, 2020, total unrecognized compensation cost related to the unvested stock-based awards was $32.1 million, which is expected to be recognized over a weighted average period of 2.7 years.

Income Taxes

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Company recognized an income tax benefit of $0 and $0.5 million during the three months ended March 31, 2020 and 2019, respectively. The income tax benefits recognized during the three months ended March 31, 2019 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 March 31, 2020 and 2019 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	3 Months Ended
Net Loss per Share	Mar. 31, 2020
Earnings Per Share [Abstract]
Net Loss per Share	10. 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 March 31, 2020 2019 Numerator: Net loss $ (14,282 ) $ (33,198 ) Denominator: Weighted average common shares outstanding—basic and diluted 60,008,217 45,004,521 Net loss per share—basic and diluted $ (0.24 ) $ (0.74 ) The Company excluded 18,507 shares and 197,696 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 March 31, 2020 and 2019, 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: Three Months Ended March 31, 2020 2019 Options to purchase common stock 10,983,227 8,012,029 Unvested restricted common stock 6,763 168,758 10,989,990 8,180,787

Leases

Leases	3 Months Ended
Leases	Mar. 31, 2020
Leases [Abstract]
Leases	11. Leases 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 which a series of cleanroom suites will be built at AMRI’s manufacturing facility (“AMRI Agreement”) in accordance with the Company’s objectives. The Company will have exclusive use of the space once the build-out build-out build-out three build-out (“Build-Out Build-Out Build-Out Build-Out build-out build-out ASU No. 2016-02, Leases (Topic 842) (“ASC 842”). As of March 31, 2020, 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 right-of-use assets or lease liabilities on the Company’s condensed consolidated balance sheet.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
Commitments and Contingencies	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. 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. In September 2018, the Company and Roche amended the agreement to remove and replace the Purchase Commitment for certain manufactured products and related raw materials supplied by Roche. The agreement, as amended, specifies a minimum purchase requirement for certain custom manufactured products. As of March 31, 2020, the Company’s purchase commitments under the agreement totaled $14.0 million, with $3.5 million committed as payments each year from 2021 to 2024. Research and development expenses related to this agreement totaled $0.7 million and $2.5 million during the three months ended March 31, 2020 and 2019, respectively. 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 March 31, 2020, the Company paid MIT $0.3 million towards the total committed amount. Research and development expenses related to this agreement during the three months ended March 31, 2020 were $0.3 million. There were no amounts payable by the Company under the agreement as of March 31, 2020. 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”). Research and development expenses related to this agreement totaled $0 million and $0.2 million during the three months ended March 31, 2020 and 2019, respectively. As amended, the 2016 MIT Agreement expired in October 2019 and as of March 31, 2020, the Company had paid MIT the total committed amount. 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, including products that may be developed under the Company’s collaboration with Sanofi. 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 $0.2 million during each of the three months ended March 31, 2020 and 2019. 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, if the Company, its affiliates or MIT receives a request from a third party to develop such licensed product for which the Company is unable to, within nine months of receiving notice of any such request, either demonstrate that the Company has initiated a fully funded project for the commercial development of such licensed product, and provide a business plan with acceptable milestones; demonstrate that the licensed product proposed by such third party would be competitive with a licensed product for which the Company has initiated a fully funded project; or enter into a sublicense agreement with such third party on commercially reasonable terms, and, in each case, MIT, in its sole discretion, grants a license to such third party for the specified patent rights. Research and development expenses related to this agreement totaled less than $0.1 million during each of the three months ended March 31, 2020 and 2019. As of March 31, 2020 and December 31, 2019, there were no liabilities recorded by the Company related to this agreement. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of March 31, 2020 and December 31, 2019. 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	3 Months Ended
Related Party Transactions	Mar. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions	13. Related Party Transactions 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.

Subsequent Event

Subsequent Event	3 Months Ended
Subsequent Event	Mar. 31, 2020
Subsequent Events [Abstract]
Subsequent Event	14. Subsequent Events Between April 1, 2020 and May 5, 2020, the Company settled transactions that occurred pursuant to the Sales Agreement with Jefferies, whereby the Company issued and sold an aggregate of 2,863,163 shares of its common stock between March 30, 2020 and May 1, 2020, resulting in gross proceeds of $37.9 million, before deducting commissions of $1.1 million and other offering expenses of $0.2 million.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2020
Accounting Policies [Abstract]
Recently Adopted Accounting Pronouncement	Recently Adopted Accounting Pronouncements In January 2017, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2017-04, Intangibles—Goodwill and Other: Simplifying the Test for Goodwill Impairment (Topic 350) In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606. Revenue from Contracts with Customers Collaborative Arrangements
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments available-for-sale In December 2019, the FASB issued ASU No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes step-up

Collaboration Agreement (Tables

Collaboration Agreement (Tables)	3 Months Ended
Collaboration Agreement (Tables)	Mar. 31, 2020
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 2020 2019 Collaboration revenue $ 4,654 $ 1,474
Balance of Contract Liabilities Related to Collaboration Agreements	The following table presents the balance of the Company’s contract liabilities (in thousands): March 31, December 31, Contract liabilities Deferred revenue $ 42,088 $ 43,356

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	3 Months Ended
Intangible Assets and Goodwill (Tables)	Mar. 31, 2020
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: March 31, 2020 Estimated Life Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 6 years $ 45,992 $ (3,439 ) $ — $ 42,553 Indefinite-lived intangible assets: IPR&D—CF Indefinite 42,291 — — 42,291 Total intangible assets, net $ 88,283 $ (3,439 ) $ — $ 84,844 December 31, 2019 Estimated Life Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 years $ 45,992 $ (2,747 ) $ — $ 43,245 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 $ (2,747 ) $ (18,559 ) $ 85,536

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	3 Months Ended
Fair Value of Financial Assets and Liabilities (Tables)	Mar. 31, 2020
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 Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 53,850 $ — $ 53,850 U.S. government agency bonds — 57,627 — 57,627 $ — $ 111,477 $ — $ 111,477 Liabilities: Contingent consideration $ — $ — $ 94,203 $ 94,203 $ — $ — $ 94,203 $ 94,203 Fair Value Measurements as of December 31, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 56,591 $ — $ 56,591 U.S. government agency bonds — 104,098 — 104,098 $ — $ 160,689 $ — $ 160,689 Liabilities: Contingent consideration $ — $ — $ 103,655 $ 103,655 $ — $ — $ 103,655 $ 103,655
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 Fair Value at Projected Year of Payment March 31, 2020 December 31, 2019 Earnout payments 2026 - 2039 87,054 $ 96,097 Milestone payments 2026 - 2030 7,149 7,558 $ 94,203 $ 103,655
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, 2019 $ 103,655 Decrease in fair value of contingent consideration (9,452 ) Balance as of March 31, 2020 $ 94,203

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
Property and Equipment, Net (Tables)	Mar. 31, 2020
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): March 31, December 31, Laboratory equipment $ 9,700 $ 9,044 Computer equipment 893 779 Office equipment 883 883 Leasehold improvements 5,635 5,635 Construction in progress 4,590 3,460 21,701 19,801 Less: Accumulated depreciation and amortization (7,920 ) (7,262 ) $ 13,781 $ 12,539

Accrued Expenses (Tables)

Accrued Expenses (Tables)	3 Months Ended
Accrued Expenses (Tables)	Mar. 31, 2020
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): March 31, December 31, Accrued external research and development expenses $ 3,879 $ 1,763 Accrued employee compensation and benefits 2,298 3,547 Accrued consultant and professional fees 1,174 1,390 Other 127 372 $ 7,478 $ 7,072

Incentive Stock Options and R_2

Incentive Stock Options and Restricted Stock (Tables)	3 Months Ended
Incentive Stock Options and Restricted Stock (Tables)	Mar. 31, 2020
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2019 (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, 2019 8,646,378 $ 8.06 8.42 $ 3,687 Granted 2,369,168 $ 7.66 Exercised (15,596 ) $ 8.49 Forfeited (16,723 ) $ 9.16 Outstanding as of March 31, 2020 10,983,227 $ 7.97 8.56 $ 22,260 Exercisable as of March 31, 2020 4,287,057 $ 7.72 7.88 $ 9,685 Vested and expected to vest as of March 31, 2020 10,983,227 $ 7.97 8.56 $ 22,260
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: Three Months Ended March 31, 2020 2019 Risk-free interest rate 0.86 % 2.47 % Expected term (in years) 6.1 6.0 Expected volatility 68.0 % 73.9 % Expected dividend yield 0 % 0 %
Summary of Restricted Stock Activity	The following table summarizes the Company’s restricted stock activity since December 31, 2019: Number of Shares Weighted Average Grant-Date Fair Value Unvested restricted common stock outstanding as of December 31, 2019 34,168 $ 1.28 Forfeited restricted common stock — $ — Vested restricted common stock (27,405 ) $ 1.28 Unvested restricted common stock outstanding as of March 31, 2020 6,763 $ 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 March 31, 2020 2019 Research and development expenses $ 1,454 $ 869 General and administrative expenses 1,718 1,090 $ 3,172 $ 1,959

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
Net Loss per Share (Tables)	Mar. 31, 2020
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 March 31, 2020 2019 Numerator: Net loss $ (14,282 ) $ (33,198 ) Denominator: Weighted average common shares outstanding—basic and diluted 60,008,217 45,004,521 Net loss per share—basic and diluted $ (0.24 ) $ (0.74 )
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: Three Months Ended March 31, 2020 2019 Options to purchase common stock 10,983,227 8,012,029 Unvested restricted common stock 6,763 168,758 10,989,990 8,180,787

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ in Thousands	1 Months Ended		3 Months Ended
	May 05, 2020USD ($)	May 01, 2020USD ($)shares	Mar. 31, 2020USD ($)Subsidiary	Mar. 31, 2019USD ($)	Mar. 13, 2020USD ($)	Dec. 31, 2019USD ($)Disease	Jul. 31, 2019USD ($)	Jul. 03, 2019USD ($)
Initial Public Offering [Line Items]
Number of wholly owned subsidiaries \| Subsidiary			2
Accumulated deficit			$ (373,778)			$ (359,496)
Net loss			(14,282)	$ (33,198)
Stock available to be sold under sales agreement		$ 62,100
Cash, cash equivalents and short-term investments			$ 155,100
Sale from time to time of equity and debt securities					$ 350,000			$ 250,000
Sanofi Pasteur Collaboration and Licensing Agreement [Member] \| Sanofi Pasteur Inc [Member]
Initial Public Offering [Line Items]
Number of infectious disease pathogens \| Disease						6
Open Market Sale Agreement [Member]
Initial Public Offering [Line Items]
Payments of other offering expenses		200
Payment of issuance commissions		$ 1,100
Net proceeds from issuance of common stock	$ 36,600
Issuance of common, shares \| shares		2,863,163
Proceeds from issuance of common stock		$ 37,900
Market sale aggreement of common stock							$ 50,000
Open Market Sale Agreement [Member] \| Maximum [Member]
Initial Public Offering [Line Items]
Market sale aggreement of common stock					100,000
Open Market Sale Agreement [Member] \| Minimum [Member]
Initial Public Offering [Line Items]
Market sale aggreement of common stock					$ 50,000

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)Disease	Mar. 31, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Period of research term	3 years
Sanofi Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Initial revenue recognizing period	8 years
Number of infectious disease pathogens for vaccine development \| Disease	5
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	$ 30.5
Non-refundable upfront payment	45
Estimated reimbursable employee cost	34.3
Estimated reimbursable development cost	88.3
Estimated milestone payments	14
Revenue recognized from contract liabilities	$ 1.3	$ 0.7
Sanofi Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognizing period	6 years
Number of infectious disease pathogens for vaccine development \| Disease	6
Receivable from collaboration	$ 805
Sales milestone payment receivable	$ 85

Collaboration Agreement - Summa

Collaboration Agreement - Summary of Collaboration Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration revenue	$ 4,654	$ 1,474

Collaboration Agreement - Balan

Collaboration Agreement - Balance of Contract Liabilities Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Contract liabilities
Deferred revenue	$ 42,088	$ 43,356

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	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, accumulated amortization	$ (3,439)	$ (2,747)
Definite-lived intangible assets, Impairment Charge		(18,559)
Total intangible assets, gross carrying amount	88,283	106,842
Total intangible assets, net carrying amount	$ 84,844	$ 85,536
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	6 years	8 years
Definite-lived intangible assets, gross carrying amount	$ 45,992	$ 45,992
Definite-lived intangible assets, accumulated amortization	(3,439)	(2,747)
Definite-lived intangible assets, net carrying amount	42,553	43,245
In-Process Research and Development [Member] \| Cystic Fibrosis [Member] \| Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross carrying amount/net carrying amount	$ 42,291	42,291
In-Process Research and Development [Member] \| OTC Deficiency [Member] \| Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, Impairment Charge		(18,559)
Indefinite-lived intangible assets, gross carrying amount/net carrying amount		$ 18,559

Intangible Assets and Goodwil_3

Intangible Assets and Goodwill - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	May 01, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Finite Lived Intangible Assets [Line Items]
indefinite lived intangible assets Impairment charge				$ 18,559,000
Goodwill, Impairment Charges		$ 0	$ 0
Goodwill		21,359,000		21,359,000
Goodwill changes	$ 0
Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Goodwill		21,400,000
Shire's MRT Program [Member] \| Sanofi Agreement [Member]
Finite Lived Intangible Assets [Line Items]
Estimated amortization expense of intangible assets for 2020		8,200,000
Estimated amortization expense of intangible assets for 2021		9,200,000
Estimated amortization expense of intangible assets for 2022		9,300,000
Estimated amortization expense of intangible assets for 2023		12,100,000
Estimated amortization expense of intangible assets for 2024		4,500,000
OTC Deficiency Program [Member]
Finite Lived Intangible Assets [Line Items]
Amortization of Intangible Assets		700,000	$ 500,000
OTC Deficiency [Member] \| Prepaid Expenses and Other Current Assets [Member]
Finite Lived Intangible Assets [Line Items]
short-term receivables		$ 900,000
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				$ 18,559,000

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	Mar. 31, 2020	Dec. 31, 2019
Assets:
Total, assets	$ 111,477	$ 160,689
Liabilities:
Total, liabilities	94,203	103,655
Fair Value, Inputs, Level 2
Assets:
Total, assets	111,477	160,689
Fair Value, Inputs, Level 3
Liabilities:
Total, liabilities	94,203	103,655
Money Market Funds
Assets:
Total, assets	53,850	56,591
Money Market Funds \| Fair Value, Inputs, Level 2
Assets:
Total, assets	53,850	56,591
U.S. Government Agency Bonds
Assets:
Total, assets	57,627	104,098
U.S. Government Agency Bonds \| Fair Value, Inputs, Level 2
Assets:
Total, assets	57,627	104,098
Contingent Consideration
Liabilities:
Total, liabilities	94,203	103,655
Contingent Consideration \| Fair Value, Inputs, Level 3
Liabilities:
Total, liabilities	$ 94,203	$ 103,655

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) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Earnout Payments \| Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2039	2039
Earnout Payments \| Maximum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2026	2026
Milestone Payments \| Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2030	2030
Milestone Payments \| Maximum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2026	2026
Contingent Consideration
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	$ 94,203	$ 103,655
Contingent Consideration \| Earnout Payments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	87,054	96,097
Contingent Consideration \| Milestone Payments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	$ 7,149	$ 7,558

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
	Mar. 31, 2020	Mar. 31, 2019
Business Acquisition, Contingent Consideration [Line Items]
Beginning Balance	$ 103,655
Discontinuation of MRT5201 and Increase in fair value of contingent consideration	(9,452)	$ 11,702
Ending Balance	94,203
Contingent Consideration
Business Acquisition, Contingent Consideration [Line Items]
Discontinuation of MRT5201 and Increase in fair value of contingent consideration	$ (9,452)

Fair Value of Financial Asset_6

Fair Value of Financial Assets and Liabilities - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2020USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 97,468,000	$ 62,431,000	$ 84,580,000
Short-term investments amortized cost	56,800,000
Short-term investments unrealized gain	900,000
Short-term investments fair value	$ 57,600,000
Contingent Consideration
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Discount Rate	14.5		13.5
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short term investments maturity period	1 year
Money Market Funds \| Fair Value, Inputs, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 53,800,000		$ 56,600,000
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
Fair value,assets transfers from Level 1 to Level 2	0		0
Fair value, liabilities transfers from Level 1 to Level 2	0		0
Fair value, liabilities transfers from Level 2 to Level 1	0		0
Fair Value, liabilities transfers into (out of) Level 3	$ 0		$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 21,701	$ 19,801
Less: Accumulated depreciation and amortization	(7,920)	(7,262)
Property and equipment, net	13,781	12,539
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	9,700	9,044
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	893	779
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	883	883
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,635	5,635
Construction In Progress [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 4,590	$ 3,460

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 0.7	$ 0.6

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 2,298	$ 3,547
Accrued external research and development expenses	3,879	1,763
Accrued consultant and professional fees	1,174	1,390
Other	127	372
Total accrued expenses	$ 7,478	$ 7,072

Incentive Stock Options and R_3

Incentive Stock Options and Restricted Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Jan. 01, 2020	Jun. 15, 2018	Mar. 07, 2018	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jan. 01, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation cost related to unvested stock-based awards				$ 32.1
Unrecognized compensation cost, period for recognition				2 years 8 months 12 days
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Intrinsic value of stock options, exercised				$ 0.1	$ 0.4
Weighted average grant-date fair value				$ 4.67	$ 5.25
2018 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares reserved for issuance				7,243,399		4,829,847
Number of shares remaining available for issuance			360,514
Increase in common stock issued	2,400,829
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			418,697				870,096
Percentage threshold of outstanding shares under the plan			1.00%
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				12,723
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

Incentive Stock Options and R_4

Incentive Stock Options and Restricted Stock - Summary of Stock Option Activity (Detail) - Stock Options [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Number of shares
Number of shares, beginning balance	8,646,378
Number of shares, granted	2,369,168
Number of shares, exercised	(15,596)
Number of shares, forfeited	(16,723)
Number of shares, ending balance	10,983,227	8,646,378
Number of shares, exercisable	4,287,057
Number of shares, vested and expected to vest, ending balance	10,983,227
Weighted average exercise price
Weighted average exercise price, beginning balance	$ 8.06
Weighted average exercise price, granted	7.66
Weighted average exercise price, exercised	8.49
Weighted average exercise price, forfeited	9.16
Weighted average exercise price, ending balance	7.97	$ 8.06
Weighted average exercise price, exercisable	7.72
Weighted average exercise price, vested and expected to vest, ending balance	$ 7.97
Weighted average remaining contractual term
Weighted average remaining contractual term, ending balance	8 years 6 months 21 days	8 years 5 months 1 day
Weighted average remaining contractual term, exercisable	7 years 10 months 17 days
Weighted average remaining contractual term, vested and expected to vest, ending balance	8 years 6 months 21 days
Intrinsic value
Intrinsic value, ending balance	$ 22,260	$ 3,687
Intrinsic value, exercisable	9,685
Intrinsic value, vested and expected to vest, ending balance	$ 22,260

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]	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	0.86%	2.47%
Expected term (in years)	6 years 1 month 6 days	6 years
Expected volatility	68.00%	73.90%
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]	3 Months Ended
	Mar. 31, 2020$ / sharesshares
Number of shares
Number of shares, beginning balance \| shares	34,168
Number of shares, forfeited \| shares	0
Number of shares, vested \| shares	(27,405)
Number of shares, ending balance \| shares	6,763
Weighted average grant-date fair value
Weighted average grant-date fair value, beginning balance \| $ / shares	$ 1.28
Weighted average grant-date fair value, forfeited \| $ / shares	0
Weighted average grant-date fair value, vested \| $ / shares	1.28
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
	Mar. 31, 2020	Mar. 31, 2019
Stock-based compensation expense	$ 3,172	$ 1,959
Research and Development Expenses [Member]
Stock-based compensation expense	1,454	869
General and Administrative Expenses [Member]
Stock-based compensation expense	$ 1,718	$ 1,090

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating Loss Carryforwards [Line Items]
Income tax benefits	$ 0	$ (486)
Shires Mrt Program [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefits	$ 0	$ 500

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
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net loss	$ (14,282)	$ (33,198)
Denominator:
Weighted average common shares outstanding—basic and diluted	60,008,217	45,004,521
Net loss per share—basic and diluted	$ (0.24)	$ (0.74)

Net Loss per Share - Additional

Net Loss per Share - Additional Information (Detail) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	10,989,990	8,180,787
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	6,763	168,758
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	18,507	197,696

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	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	10,989,990	8,180,787
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	10,983,227	8,012,029
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	6,763	168,758

Leases - Additional Information

Leases - Additional Information (Details) - Suite Retention And Development Agreement [Member] $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)
Lessee Lease Description [Line Items]
Commitment to Build Out Cost	$ 6
Shared Overage cost commitment	11
Albany Molecular Research, Inc. ("AMRI") [Member]
Lessee Lease Description [Line Items]
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
Operating Lease Renewal Term	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] \| Other Noncurrent Assets [Member]
Lessee Lease Description [Line Items]
Build Out Costs paid	$ 5.8
Albany Molecular Research, Inc. ("AMRI") [Member] \| Prepaid Expenses and Other Current Assets [Member]
Lessee Lease Description [Line Items]
Payments For Purchase Of Deliverables	$ 1

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	Sep. 30, 2019	Nov. 30, 2016	Mar. 31, 2020	Mar. 31, 2019
Loss Contingencies [Line Items]
Research and development			$ 21,439	$ 17,423
Research agreement, expiration period			2022-12
Research agreement, payment			$ 300
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			$ 14,000
Purchase commitments, year 2021			3,500
Purchase commitments, year 2022			3,500
Purchase commitments, year 2023			3,500
Purchase commitments, year 2024			3,500
Research and development			700	2,500
MIT Research Agreement [Member]
Loss Contingencies [Line Items]
Research and development			300
Research agreement, committed amount	$ 4,100	$ 3,100
Research agreement, expiration period	2022-12	2019-10
Research agreement, payment			0	200
MIT Exclusive Patent License Agreement [Member]
Loss Contingencies [Line Items]
Research and development			100	100
Annual license maintenance payments			200
Payments of annual license maintenance fees			200	$ 200
MIT Exclusive Patent License Agreement [Member] \| Sanofi Pasteur Inc [Member]
Loss Contingencies [Line Items]
Upfront payment received			700
MIT Exclusive Patent License Agreement [Member] \| Milestone Payment One [Member]
Loss Contingencies [Line Items]
License agreement, milestone payments			1,375
MIT Exclusive Patent License Agreement [Member] \| Milestone Payment Two [Member]
Loss Contingencies [Line Items]
License agreement, milestone payments			$ 1,250

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - Baupost Group LLC [Member] - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended
	Sep. 30, 2019	Mar. 31, 2020
Private Placement [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
Sale of Stock, Consideration Received on Transaction		$ 20
Public Offering [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
Sale of Stock, Consideration Received on Transaction	$ 50

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) - Private Placement [Member] $ in Millions	1 Months Ended
	May 05, 2020USD ($)shares
Subsequent Event [Line Items]
Number of stock issued and sold \| shares	2,863,163
Gross proceeds from issuance of private placement	$ 37.9
Placement agent fees	1.1
Other estimated offering expenses	$ 0.2