Rubius Therapeutics (RUBY) 10-Q 13 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Document and Entity Information
Entity Registrant Name	Rubius Therapeutics, Inc.
Entity Central Index Key	1,709,401
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		79,044,166
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 354,786	$ 104,288
Investments	54,088
Prepaid expenses and other current assets	3,492	700
Restricted cash	122
Total current assets	412,488	104,988
Property, plant and equipment, net	39,208	2,415
Restricted cash	1,102	284
Total assets	452,798	107,687
Current liabilities:
Accounts payable	4,542	2,033
Accrued expenses and other current liabilities	10,245	2,986
Current portion of long-term debt	1,222	2,139
Total current liabilities	16,009	7,158
Long-term debt, net of discount and current portion	4,245	3,302
Deferred rent	144	158
Preferred stock warrant liability		866
Liability for early exercise of stock options and restricted stock	187	100
Lease liability, net of current portion	21,206
Total liabilities	41,791	11,584
Commitments and contingencies (Note 11)
Convertible preferred stock (Series A, B and C), $0.001 par value; no shares and 44,070,808 shares authorized at September 30, 2018 and December 31, 2017, respectively, no shares and 43,933,006 shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively; liquidation preference of $138,242 at December 31, 2017		139,790
Stockholders' equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at September 30, 2018 and December 31, 2017, respectively; no shares issued or outstanding at September 30, 2018 and December 31, 2017
Common stock, $0.001 par value; 150,000,000 shares and 65,000,000 shares authorized at September 30, 2018 and December 31, 2017, respectively, 79,042,603 shares and 14,977,317 shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively	79	15
Additional paid-in capital	533,846	17,277
Accumulated other comprehensive loss	(19)
Accumulated deficit	(122,899)	(60,979)
Total stockholders' equity (deficit)	411,007	(43,687)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 452,798	$ 107,687

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Preferred stock, par value (in dollars per share)	$ 0.001
Preferred stock, share authorized (in shares)	10,000,000
Preferred stock, shares issued (in shares)	0
Preferred stock, shares outstanding (in shares)	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, share authorized (in shares)	150,000,000	65,000,000
Common stock, shares issued (in shares)	79,042,603	14,977,317
Common stock, shares outstanding (in shares)	79,042,603	14,977,317
Convertible preferred stock
Convertible preferred stock, par value (in dollars per share)		$ 0.001
Convertible preferred stock, shares authorized ( in shares)		44,070,808
Convertible preferred stock, shars issued (in shares)		43,933,006
Convertible preferred stock, shars outstanding (in shares)		43,933,006
Convertible preferred stock, liquidation preference		$ 138,242

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Research and development	$ 14,363	$ 6,126	$ 35,230	$ 14,628
General and administrative	13,191	5,849	27,311	11,163
Total operating expenses	27,554	11,975	62,541	25,791
Loss from operations	(27,554)	(11,975)	(62,541)	(25,791)
Other income (expense):
Change in fair value of preferred stock warrant liability	(426)	(59)	(2,187)	(576)
Interest expense	(87)	(90)	(259)	(212)
Interest income and other income (expense), net	1,705	205	2,975	249
Total other income (expense), net	1,192	56	529	(539)
Net loss	(26,362)	(11,919)	(62,012)	(26,330)
Accretion of Series A redeemable convertible preferred stock to redemption value				(656)
Net loss attributable to common stockholders	$ (26,362)	$ (11,919)	$ (62,012)	$ (26,986)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.42)	$ (1.48)	$ (2.33)	$ (3.42)
Weighted average common shares outstanding, basic and diluted	62,311,111	8,078,196	26,662,233	7,899,507
Comprehensive loss:
Net loss	$ (26,362)	$ (11,919)	$ (62,012)	$ (26,330)
Other comprehensive income (loss):
Unrealized gains (losses) on investments, net of tax of $0	8		(19)
Comprehensive loss	$ (26,354)	$ (11,919)	$ (62,031)	$ (26,330)

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Parenthetical) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS
Unrealized gains (losses) on investments, tax	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (62,012)	$ (26,330)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	18,402	8,807
Depreciation and amortization expense	879	251
Change in fair value of preferred stock warrant liability	2,187	576
Accretion of discount on investments	(198)
Non-cash interest expense	26	30
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,792)	(68)
Accounts payable	2,402	2,017
Accrued expenses and other current liabilities	2,472	1,825
Deferred rent	(14)	(6)
Net cash used in operating activities	(38,648)	(12,898)
Cash flows from investing activities:
Purchases of property, plant and equipment	(11,671)	(1,257)
Purchases of investments	(87,369)
Sales and maturities of investments	33,460
Net cash used in investing activities	(65,580)	(1,257)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	100,986	120,117
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	257,866
Payments of initial public offering costs	(3,449)
Proceeds from issuance of common stock upon exercise of stock options	18	29
Proceeds from sale of restricted common stock		100
Proceeds from repayment of promissory notes	245
Payments of debt issuance costs		(11)
Proceeds from borrowings under loan and security agreement		1,500
Net cash provided by financing activities	355,666	121,735
Net increase in cash, cash equivalents and restricted cash	251,438	107,580
Cash, cash equivalents and restricted cash at beginning of period	104,572	7,068
Cash, cash equivalents and restricted cash at end of period	356,010	114,648
Supplemental disclosure of non-cash investing and financing information:
Accretion of Series A redeemable convertible preferred stock to redemption value		656
Purchases of property, plant and equipment included in accounts payable or accrued expenses	1,353	443
Issuance of preferred stock warrant in connection with loan and security agreement		14
Amounts capitalized under build-to-suit lease transaction	24,657
Offering costs and issuance costs included in accounts payable and accrued expenses	99	$ 50
Reclassification of warrants to additional paid-in capital	3,053
Conversion of preferred stock to common stock upon closing of the initial public offering	$ 240,776

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2018
Nature of the Business and Basis of Presentation
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Rubius Therapeutics, Inc. (“Rubius” or the “Company”) is a therapeutics company focused on using its platform to develop red cell therapeutics for the treatment of patients with severe diseases. Rubius was incorporated in April 2013 as VL26, Inc. under the laws of the State of Delaware. In January 2015, the Company changed its name to Rubius Therapeutics, Inc. The Company is subject to risks and uncertainties 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, the ability to establish clinical- and commercial-scale manufacturing processes and the ability to secure additional capital to fund operations. In addition, the Company is subject to uncertainty regarding the performance and safety of red cell therapeutics in humans. Product candidates currently under development will require significant additional research and development efforts, including extensive 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 drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On July 20, 2018, the Company completed its initial public offering (“IPO”), pursuant to which it issued and sold 12,055,450 shares of common stock, inclusive of 1,572,450 shares sold by the Company pursuant to the full exercise of the underwriters’ option to purchase additional shares. The aggregate net proceeds received by the Company from the IPO were $254.3 million, after deducting underwriting discounts and commissions and other offering costs. Upon the closing of the IPO, all of the shares of the Company’s outstanding convertible preferred stock then outstanding automatically converted into 51,845,438 shares of common stock (see Note 7). The accompanying condensed consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has funded its operations with proceeds from sales of convertible preferred stock and borrowings under a loan and security agreement, and most recently, with proceeds from the IPO completed in July 2018. The Company has incurred recurring losses since inception, including net losses of $62.0 million for the nine months ended September 30, 2018 and $43.8 million for the year ended December 31, 2017. As of September 30, 2018, the Company had an accumulated deficit of $122.9 million. The Company expects to continue to generate operating losses in the foreseeable future. As of November 13, 2018, the issuance date of the interim consolidated financial statements, the Company expects that its cash, cash equivalents and investments will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the issuance date of the interim consolidated financial statements. The Company’s consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany accounts and transactions have been eliminated in consolidation.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Unaudited Interim Financial Information The consolidated balance sheet at December 31, 2017 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited consolidated financial statements as of September 30, 2018 and for the three and nine months ended September 30, 2018 and 2017 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These 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, 2017 included in the Company’s Registration Statement on Form S-1, as amended, File No. 333-225840 on file with the SEC. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s consolidated financial position as of September 30, 2018 and consolidated results of operations for the three and nine months ended September 30, 2018 and 2017 and the consolidated cash flows for the nine months ended September 30, 2018 and 2017, have been made. The Company’s consolidated results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the valuation of common stock prior to the IPO, the valuation of stock-based awards and the valuation of the preferred stock warrant liability prior to the IPO. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. As of September 30, 2018 and December 31, 2017, the Company did not maintain cash balances in excess of federally insured limits. The Company’s cash equivalents, as of September 30, 2018, consisted of U.S. government money market funds, U.S. government agency bonds and U.S. government treasury notes. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and raw materials for its development programs. These programs could be adversely affected by a significant interruption in these manufacturing services or the availability of raw materials. Restricted Cash As of September 30, 2018 and December 31, 2017, the Company maintained letters of credit totaling $1.2 million and $0.3 million, respectively, for the benefit of the landlords of its leased properties. The Company was required to maintain separate cash balances of these amounts to secure the letters of credit. Related to these separate cash balances, the Company classified $1.1 million and $0.3 million as restricted cash (non-current) in its consolidated balance sheet as of September 30, 2018 and December 31, 2017, respectively, and classified $0.1 million as restricted cash (current) in its consolidated balance sheet as of September 30, 2018. Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Investments The Company’s investments are classified as available-for-sale and are carried at fair value, with the unrealized gains and losses reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value determined to be other than temporary are based on the specific identification method and are included as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company classifies its investments with maturities beyond one year as short-term, based on their highly liquid nature and because such investments are available for current operations. The Company evaluates its investments with unrealized losses for other-than-temporary impairment. When assessing investments for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: · Level 1—Quoted prices in active markets for identical assets or liabilities. · Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. · Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s long-term debt approximates its fair value due to its variable interest rate, which approximates a market interest rate. Recently Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board, (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Compensation — Stock Compensation (Topic 718), Improvements to Non-Employee Share-Based Payment Accounting (“ASU 2018-07”) . This ASU is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. For public entities, ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For non-public entities, ASU 2018-07 is effective for annual periods beginning after December 15, 2019. Early adoption is permitted for all entities but no earlier than the Company’s adoption of ASU 2014-09. The Company adopted ASU 2018-07 effective January 1, 2018 by remeasuring outstanding equity-classified awards issued to non-employees for which a measurement date had not been established as of January 1, 2018 through a cumulative-effect adjustment to accumulated deficit as of January 1, 2018. The Company has elected to estimate the expected term of options utilizing the simplified method for both employee and non-employee options that qualify as plain-vanilla options. The Company has elected to account for forfeitures for non-employee options as they occur rather than apply an estimated forfeiture rate to stock-based compensation expense . The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of ASU 2018-07 on January 1, 2018 (in thousands): Balance at Balance at December 31, 2017 Adjustments January 1, 2018 Additional paid-in capital $ 17,277 $ (92) $ 17,185 Accumulated deficit $ (60,979) $ 92 $ (60,887) The $0.1 million adjustment is the result of the change in fair value of the unvested awards, representing awards for which a measurement date had not been established, using an expected term rather than the contractual term of the awards. As a result of adopting ASU 2018-07, the results for the three months ended March 31, 2018 and three and six months ended June 30, 2018 have been revised to reflect the adoption of ASU 2018-07 effective January 1, 2018. These revisions will be reflected in future filings containing such financial information. The following tables summarize the impact of adoption to the Company’s previously issued consolidated balance sheets and consolidated statements of operations and comprehensive loss (in thousands): Consolidated Balance Sheets As of March 31, 2018 As of June 30, 2018 As previously As previously reported As revised reported As revised Additional paid-in capital $ 20,738 $ 19,802 $ 34,636 $ 25,840 Accumulated deficit $ (76,234) $ (75,298) $ (105,333) $ (96,537) Consolidated Statements of Operations and Comprehensive Loss Three Months Ended March 31, 2018 Three Months Ended June 30, 2018 Six Months Ended June 30, 2018 As previously As previously As previously reported As revised reported As revised reported As revised Operating expenses: Research and development $ 9,650 $ 9,506 $ 11,965 $ 11,361 $ 21,615 $ 20,867 General and administrative 5,797 5,097 16,279 9,023 22,076 14,120 Total operating expenses 15,447 14,603 28,244 20,384 43,691 34,987 Loss from operations (15,447) (14,603) (28,244) (20,384) (43,691) (34,987) Net loss attributable to common stockholders $ (15,255) $ (14,411) $ (29,099) $ (21,239) $ (44,354) $ (35,650) Net loss per share attributable to common stockholders, basic and diluted $ (1.83) $ (1.72) $ (3.33) $ (2.43) $ (5.19) $ (4.17) Comprehensive loss: Net loss (15,255) (14,411) (29,099) (21,239) (44,354) (35,650) Other comprehensive income (loss): Unrealized gains (losses) on investments, net of tax of $0 (45) (45) 18 18 (27) (27) Comprehensive loss $ (15,300) $ (14,456) $ (29,081) $ (21,221) $ (44,381) $ (35,677) As of the adoption date of January 1, 2018, the Company had 330,917 outstanding options to non-employees for which a measurement date had not been established. As of the adoption date of January 1, 2018, the Company had 4,767,014 shares of restricted common stock held by non-employees that were being accounted for as stock options for which a measurement date had not been established (see Note 10). The weighted average fair value of these awards was $5.88 per share as of January 1, 2018. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of outstanding awards granted to non-employees for which a measurement date had not been established as of the adoption date of January 1, 2018: Risk-free interest rate 2.3 % Expected volatility 74 % Expected dividend yield — Expected term (in years) 6.1 Common stock value $ 6.28 Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. For public entities, the guidance is effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. For nonpublic entities, the guidance is effective for annual reporting periods beginning after December 15, 2019 and for interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted for all entities. ASU 2016-02 initially required adoption using a modified retrospective approach, under which all years presented in the financial statements would be prepared under the revised guidance. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842) , which added an optional transition method under which financial statements may be prepared under the revised guidance for the year of adoption, but not for prior years. Under the latter method, entities will recognize a cumulative catch-up adjustment to the opening balance of retained earnings in the period of adoption. The Company is currently evaluating whether to early-adopt ASU 2016-02 and evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements. In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception (“ASU 2017-11”). Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within Accounting Standards Codification (“ASC”) Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. For public entities, this guidance is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For nonpublic entities, this guidance is effective for annual periods beginning after December 15, 2019 and for interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted for all entities, including adoption in an interim period. If an entity early adopts the amendments in an interim period, any adjustments should be reflected as of the beginning of the fiscal year that includes that interim period. The Company is currently evaluating whether to early-adopt ASU 2017-11 and evaluating the impact that the adoption of ASU 2017-11 will have on its consolidated financial statements.

Investments and Fair Value of F

Investments and Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2018
Investments and Fair Value of Financial Assets and Liabilities
Investments and Fair Value of Financial Assets and Liabilities	3. Investments and Fair Value of Financial Assets and Liabilities Investments by security type consisted of the following (in thousands): September 30, 2018 Gross Gross Unrealized Unrealized Amortized Cost Gains Losses Fair Value U.S. treasury notes (due within one year) $ 41,154 $ — $ (15) $ 41,139 U.S. government agency bonds (due within one year) 10,453 — (4) 10,449 $ 51,607 $ — $ (19) $ 51,588 The Company did not have any investments as of December 31, 2017. The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements at September 30, 2018 Using: Not Subject to Level 1 Level 2 Level 3 Leveling Total Assets: Cash equivalents: Money market funds $ 354,786 $ — $ — $ — $ 354,786 Investments: U.S. government agency bonds — 10,449 — — 10,449 U.S. treasury notes — 41,139 — — 41,139 Receivable from unsettled maturity of a security (1) — — — $ 2,500 2,500 $ 354,786 $ 51,588 $ — $ 2,500 $ 408,874 (1) Represents a receivable from the maturity of a security, which ocurred during the three months ended September 30, 2018 and for which cash settlement was received in October 2018. Fair Value Measurements at December 31, 2017 Using: Not Subject to Level 1 Level 2 Level 3 Leveling Total Assets: Cash equivalents: Money market funds $ 104,288 $ — $ — $ — $ 104,288 Liabilities: Preferred stock warrant liability $ — $ — $ 866 $ — $ 866 Money market funds were valued by the Company based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. U.S. treasury notes and U.S. government agency bonds were valued by the Company using quoted prices in active markets for similar securities, which represent a Level 2 measurement within the fair value hierarchy. There have been no changes to the valuation methods during the nine months ended September 30, 2018. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers between Level 1, Level 2 or Level 3 during the nine months ended September 30, 2018. The preferred stock warrant liability in the table above consisted of the fair value of warrants to purchase Series A and Series B convertible preferred stock (see Note 8) and was based on significant inputs not observable in the market, which represent a Level 3 measurement within the fair value hierarchy. The Company’s valuation of the preferred stock warrants utilized the Black-Scholes option-pricing model, which incorporated assumptions and estimates to value the preferred stock warrants. The Company assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. Changes in the fair value of the preferred stock warrants were recognized as other income (expense) in the consolidated statements of operations and comprehensive loss. Upon the closing of the IPO, the warrants for the purchase of preferred stock automatically became warrants for the purchase of common stock and the Company reclassified the carrying value of the warrants from a non-current liability to additional paid-in capital in its consolidated balance sheet. The following table provides a roll-forward of the aggregate fair values of the Company’s preferred stock warrants for which fair value was determined by Level 3 inputs (in thousands): Preferred stock warrant liability Fair value at December 31, 2017 $ 866 Change in fair value through the exercise date 2,187 Reclassification to additional paid-in capital in connection with IPO (3,053) Fair value at September 30, 2018 $ —

Property, Plant and Equipment,

Property, Plant and Equipment, Net	9 Months Ended
	Sep. 30, 2018
Property, Plant and Equipment, Net.
Property, Plant and Equipment, Net	4. Property, Plant and Equipment, Net Property, plant and equipment, net consisted of the following (in thousands): September 30, 2018 December 31, 2017 Laboratory equipment $ 6,151 $ 2,751 Land 1,300 — Leasehold improvements 117 117 Computer equipment 139 57 Construction-in-progress 32,964 74 40,671 2,999 Less: Accumulated depreciation and amortization (1,463) (584) $ 39,208 $ 2,415 Manufacturing Facility On July 31, 2018, the Company completed its purchase of a 135,000 square foot manufacturing facility located in Smithfield, Rhode Island for a purchase price of $8.0 million. In August 2018, the Company began renovations to customize this facility to manufacture clinical supply of its product candidates. Of the total purchase price, $1.3 million was allocated to the value of land acquired and the remaining $6.7 million was allocated to construction in progress, as the building was not ready for its intended use. During the three months ended September 30, 2018, the Company capitalized approximately $1.0 million in construction-in-progress for design and demolition costs related to the renovation project. Construction-in-progress, as of September 30, 2018, also included $24.7 million capitalized in connection with the Company’s build-to-suit lease accounting (see Note 11) and $0.5 million for the Company's share of the costs of contruction-in-progress for leasehold improvements related to the Company’s January 2018 lease for office and laboratory space in Cambridge, Massachusetts.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2018
Accrued Expenses and Other Current Liabilities
Accrued Expenses and Other Current Liabilities	5. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): September 30, 2018 December 31, 2017 Accrued employee compensation and benefits $ 1,977 $ 1,339 Accrued external research and development expenses 2,171 1,230 Accrued lease liability, current portion 3,451 — Accrued professional fees 990 219 Accrued construction-in-progress 1,336 — Other 320 198 $ 10,245 $ 2,986

Debt

Debt	9 Months Ended
	Sep. 30, 2018
Debt
Debt	6. Debt The Company is party to a loan and security agreement, as amended (the “2015 Credit Facility”), under which the Company had borrowed an aggregate of $5.5 million. Until May 2018, borrowings under the 2015 Credit Facility bore interest at an annual rate equal to the bank’s prime rate plus 1.25%, subject to a floor of 4.5%, and were repayable in monthly interest-only payments through May 2018 and in equal monthly payments of principal plus accrued interest from June 2018 until the maturity date in November 2019. In May 2018, the Company further amended the 2015 Credit Facility to modify the interest rate and extend the interest-only payment period and the maturity date. Subsequent to this amendment, outstanding borrowings under the 2015 Credit Facility bear interest at an annual rate equal to the bank’s prime rate plus 0.75%, subject to a floor of 5.5%, and are repayable in monthly interest-only payments through May 2019 and in equal monthly payments of principal plus accrued interest from June 2019 until the maturity date in November 2020. As of September 30, 2018, the interest rate applicable to borrowings under the 2015 Credit Facility was 6.00%. The May 2018 amendment to the 2015 Credit Facility was accounted for as a debt modification, rather than a debt extinguishment, based on a comparison of the present value of the cash flows under the terms of the debt immediately before and after the amendment, which resulted in a change of less than 10%. As a result, issuance costs paid to the lender in connection with the amendment were recorded as a reduction of the carrying amount of the debt liability and were not significant. Unamortized issuance costs as of the date of the modification will be amortized to interest expense using the effective interest method over the revised repayment term. Issuance costs paid to third parties were recorded as expense and were not significant. Borrowings under the 2015 Credit Facility are collateralized by substantially all of the Company’s personal property, other than its intellectual property. There are no financial covenants associated with the 2015 Credit Facility; however, the Company is subject to certain affirmative and negative covenants restricting the Company’s activities, including limitations on dispositions, mergers or acquisitions; encumbering its intellectual property; incurring indebtedness or liens; paying dividends; making certain investments; and engaging in certain other business transactions. The obligations under the 2015 Credit Facility are subject to acceleration upon the occurrence of specified events of default, including a material adverse change in the Company’s business, operations or financial or other condition. As of September 30, 2018, the estimated future principal payments due were as follows (in thousands): Year Ending December 31, 2018 (three months ending December 31) $ — 2019 2,139 2020 3,361 $ 5,500

Convertible Preferred Stock

Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2018
Convertible Preferred Stock
Convertible Preferred Stock	7. Convertible Preferred Stock The Company had issued Series A redeemable convertible preferred stock (the “Series A Preferred Stock”), Series B convertible preferred stock (the “Series B Preferred Stock”) and Series C convertible preferred stock (the “Series C Preferred Stock”). The Series A Preferred Stock, the Series B Preferred Stock and the Series C Preferred Stock are referred to collectively as the “Preferred Stock”. Upon issuance of the Series A Preferred Stock, the holders of such shares were entitled to receive cumulative dividends of 8.0% per year, compounding annually, and such shares were redeemable at the option of the holder after five years from issuance date of the Series A Preferred Stock. In connection with the issuance and sale of Series B Preferred Stock, the holders of Series A Preferred Stock agreed to remove the cumulative dividend rights and redemption features of the Series A Preferred Stock. The change to the terms of the Series A Preferred Stock was accounted for as a modification, rather than an extinguishment, of the Series A Preferred Stock based on a comparison of the fair value of the stock immediately before and after the change in terms, which resulted in a fair value change of less than 10%. This modification did not have any impact on the Company’s consolidated financial statements. For periods after the June 2017 date of the modification of the Series A Preferred Stock, the Company no longer accretes the carrying value of the Series A Preferred Stock to redemption value as such shares are no longer redeemable. In June 2017, the Company issued and sold 14,362,344 shares of Series B Preferred Stock at a price of $8.39 per share for gross proceeds of $120.5 million. The Company incurred issuance costs in connection with the Series B Preferred Stock of $0.4 million. In February 2018, the Company issued and sold 7,912,432 shares of Series C Preferred Stock at a price of $12.79 per share for gross proceeds of $101.2 million. The Company incurred issuance costs in connection with the Series C Preferred Stock of $0.2 million. Upon the closing of the IPO in July 2018, all of the shares of the Company’s outstanding convertible preferred stock automatically converted into shares of common stock.

Warrants to Purchase Convertibl

Warrants to Purchase Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2018
Warrants to Purchase Convertible Preferred Stock
Warrants to Purchase Convertible Preferred Stock	8. Warrants to Purchase Convertible Preferred Stock The Company had outstanding warrants issued in 2015 to purchase up to 133,333 shares of Series A Preferred Stock in connection with the 2015 Credit Facility (see Note 6). The warrants were exercisable at a price of $0.60 per share and had a contractual term of ten years from issuance. In May 2017, the Company issued warrants to purchase up to 2,234 shares of Series B Preferred Stock in connection with an amendment to the 2015 Credit Facility (see Note 6). The warrants were exercisable at a price of $8.39 per share and had a contractual term of ten years from issuance. The fair value of the warrants on the issuance date of less than $0.1 million was recorded as a debt discount and as a preferred stock warrant liability. The Company remeasured the fair value of the liability for these preferred stock warrants at each reporting date and recorded any adjustments as other income (expense). The warrants outstanding at each reporting date were remeasured using the Black-Scholes option-pricing model (see Note 3), and the resulting change in fair value was recorded in other income (expense) in the Company’s consolidated statements of operations and comprehensive loss. Upon the closing of the IPO in July 2018, the Company’s outstanding warrants to purchase Preferred Stock automatically became warrants to purchase an aggregate of 135,567 shares of common stock. In July 2018, the holders of such warrants completed a cashless exercise of the warrants, resulting in the Company’s issuance of 131,273 shares of common stock, whereby 4,294 shares of common stock were withheld by the Company to pay for the exercise price of the warrants (see Note 9).

Equity

Equity	9 Months Ended
	Sep. 30, 2018
Equity
Equity	9. Equity Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors. In February 2018, the Company increased the number of authorized shares of common stock from 65,000,000 shares to 75,000,000 shares. In April 2018, the Company increased the number of authorized shares of common stock from 75,000,000 shares to 78,800,000 shares. In June 2018, the Company increased the number of authorized shares of common stock from 78,800,000 shares to 79,000,000 shares. On July 20, 2018, the Company filed a restated certificate of incorporation in the State of Delaware, which, among other things, restated the number of shares of all classes of stock that the Company has authority to issue to 160,000,000 shares, consisting of (i) 150,000,000 shares of common stock, $0.001 par value per share, and (ii) 10,000,000 shares of preferred stock, $0.001 par value per share. The shares of preferred stock are currently undesignated. In July 2018, the holders of warrants to purchase 135,567 shares of common stock completed a cashless exercise of the warrants, resulting in the Company’s issuance of 131,273 shares of common stock, whereby 4,294 shares of common stock were withheld by the Company to pay for the exercise price of the warrants.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2018
Stock-Based Compensation
Stock-Based Compensation	10. Stock-Based Compensation 2014 Stock Incentive Plan The Company’s 2014 Stock Incentive Plan (the “2014 Plan”) provides for the Company to sell or issue incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2014 Plan is administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated. The total number of shares of common stock that could have been issued under the 2014 Plan was 19,152,328 shares, of which 47,447 shares remained available for future issuance prior to the effectiveness of the Company’s 2018 Stock Option and Incentive Plan (the “2018 Plan”). Upon effectiveness of the 2018 Plan in July 2018, the remaining shares available under the 2014 Plan ceased to be available for issuance and no future issuances will be made under the 2014 Plan. The shares of common stock underlying outstanding awards under the 2014 Plan that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) will be added to the shares of common stock available for issuance under the 2018 Plan. 2018 Equity Incentive Plan On July 6, 2018, the Company’s board of directors adopted and its stockholders approved the 2018 Plan, which became effective on July 16, 2018. The 2018 Plan provides for the grant of incentive stock options, non-qualified options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2018 Plan is 5,708,931, which shall be cumulatively increased on January 1, 2019 and each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors. The shares of common stock underlying any awards that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) under the 2018 Plan and the 2014 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan. 2018 Employee Stock Purchase Plan On July 6, 2018, the Company’s board of directors adopted and its stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”), which became effective on July 16, 2018. A total of 951,488 shares of common stock were reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on January 1, 2019, and each January 1 thereafter through January 1, 2028, by the least of (i) 951,488 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares as determined by the administrator of the Company’s ESPP. Stock Option Valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock-based awards granted to employees, directors and non-employees during the three and nine months ended September 30, 2018: Three Months Ended September 30, Nine Months Ended September 30, 2018 2018 Risk-free interest rate 2.82 % 2.70 % Expected volatility 74 % 74 % Expected dividend yield — — Expected term (in years) 6.4 6.2 During the nine months ended September 30, 2018, the Company granted options for the purchase of 10,122,023 shares of common stock with a weighted average exercise price of $11.98 per share and a weighted average grant-date fair value of $8.64 per share. In April 2017, an executive officer early exercised an option to purchase 1,400,000 shares of common stock, at an exercise price of $0.18 per share, for cash proceeds of $0.1 million and a promissory note for $0.2 million. The employee received shares of restricted common stock upon such exercise. The unvested shares of restricted common stock issued upon exercise are subject to the Company’s repurchase right at the lesser of the original exercise price per share or the fair value of such shares on the repurchase date. The $0.1 million of cash proceeds from the early exercise of this stock option was recorded as a liability in the Company’s consolidated balance sheet and will be reclassified to stockholders’ equity (deficit) as the shares vest and the Company’s repurchase rights related to such shares lapse. The promissory note was partial-recourse, but was treated as nonrecourse for accounting purposes. As a result, (i) this early exercise of common stock with a promissory note continued to be accounted for as an outstanding stock option and (ii) no receivable for amounts due under the promissory note was recorded on the Company’s consolidated balance sheet. Stock-based compensation expense related to this award is being recognized over the requisite service period of the award based on the grant-date fair value of the award, which was determined using the Black-Scholes option-pricing model. On June 21, 2018, the principal balance of $0.2 million and all interest that had accrued thereon, totaling less than $0.1 million, was repaid in full by the executive officer and the promissory note was terminated (see Note 13). Stock-based compensation expense for the three and nine months ended September 30, 2018 includes $2.1 million of stock-based compensation expense related to options for the purchase of an aggregate of 447,000 shares of common stock that have performance based vesting conditions for which the performance condition was achieved during the three months ended September 30, 2018. As of September 30, 2018, the Company has outstanding options for the purchase of an aggregate of 232,500 shares of common stock with performance based vesting conditions whereby the achievement of the conditions has not yet been determined to be probable, therefore the Company has not recorded any compensation expense related to these stock options. Restricted Common Stock The Company has granted restricted common stock with service-based vesting conditions. Shares of unvested restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. In April 2017, the Company issued 460,000 shares of restricted common stock, at a price of $0.19 per share, to an executive officer in exchange for a promissory note in the principal amount of $0.1 million. The promissory note was partial-recourse, but was treated as nonrecourse for accounting purposes and, as such, (i) this purchase of common stock with a promissory note was accounted for as if it were a stock option grant and (ii) no receivable for amounts due under the promissory note was recorded on the Company’s consolidated balance sheet. Stock-based compensation expense related to this award is being recognized over the requisite service period of the award based on the grant-date fair value of the award, which was determined using the Black-Scholes option-pricing model. On June 21, 2018, the principal balance of $0.1 million and all interest that had accrued thereon, totaling less than $0.1 million, was repaid in full by the executive officer and the promissory note was terminated (see Note 13). In January 2017 and May 2017, the Company issued 3,667,014 shares and 1,100,000 shares, respectively, of restricted common stock at prices of $0.19 per share and $1.65 per share, respectively, to the chairman of the Company’s board of directors in exchange for two promissory notes totaling $2.5 million. The promissory notes are partial-recourse, but were treated as nonrecourse for accounting purposes and, as such, (i) each of these purchases of common stock with a promissory note was accounted for as if it were a stock option grant and (ii) no receivable for amounts due under the promissory note was recorded on the Company’s consolidated balance sheet. All of the stock-based awards issued to the chairman of the Company’s board of directors were issued for his services as a consultant and prior to the adoption of ASU 2018-07 effective January 1, 2018 were being accounted for as non-employee stock-based awards. As a result, stock-based compensation expense related to the awards was being recognized over the requisite service period of the award based on the remeasured fair value of the award at each reporting period until the award vested, which was determined using the Black-Scholes option-pricing model. Upon the adoption of ASU 2018-07, the Company valued the remaining unvested options issued to non-employees as of January 1, 2018 and is recognizing stock-based compensation over the remaining vesting period. Effective January 1, 2018, the Company no longer remeasures the fair value of options granted to non-employees at each reporting period end (see Note 2). On June 21, 2018, the aggregate principal balance of both promissory notes of $2.5 million and all interest that had accrued thereon, totaling $0.1 million, was forgiven by the Company and the promissory notes were terminated (see Note 13). The forgiveness of these promissory notes by the Company was treated as an option modification and resulted in the recognition of incremental stock-based compensation expense of $0.2 million and $1.4 million during the three months and nine months ended September 30, 2018, respectively, which represents the change in the fair value of the award on the modification date. Stock-based compensation expense related to these awards will continue to be recognized over the requisite service period of the awards. Stock-Based Compensation The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development expenses $ 1,347 $ 170 $ 2,331 $ 1,390 General and administrative expenses 8,540 3,967 16,071 7,417 $ 9,887 $ 4,137 $ 18,402 $ 8,807 As of September 30, 2018, total unrecognized compensation cost related to the unvested stock-based awards was $93.4 million, which is expected to be recognized over a weighted average period of 3.0 years.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies
Commitments and Contingencies	11. Commitments and Contingencies Operating Leases The Company leases its office and laboratory facilities in Cambridge, Massachusetts under two noncancelable operating leases that expire in December 2018 and September 2021. The lease agreements include lease incentives, payment escalations and rent holidays, which are accrued or deferred as appropriate such that rent expense for each lease is recognized on a straight-line basis over the terms of occupancy. Rent expense for the three and nine months ended September 30, 2018 was $0.4 million and $1.1 million, respectively. Rent expense for the three and nine months ended September 30, 2017 was $0.3 million and $0.6 million, respectively. Build-to-Suit Lease In January 2018, the Company entered into a lease for office and laboratory space in Cambridge, Massachusetts. The lease term is expected to commence in late December 2018 and expires eight years from the commencement date. The Company is entitled to one five-year option to extend. The initial annual base rent is approximately $3.8 million, and such amount will increase during the initial term by 3% annually on the anniversary of the commencement date. The Company is obligated to pay its portion of real estate taxes and costs related to the premises, including costs of operations, maintenance, repair, replacement and management of the new leased premises. In connection with the lease, the Company maintains a letter of credit for the benefit of the landlord in the amount of $0.9 million, which is secured by a cash deposit of the same amount. The lease agreement allows for a landlord-provided tenant improvement allowance of $9.4 million to be applied to the costs of the construction of the leasehold improvements. The Company is not the legal owner of the leased space. However, in accordance with ASC 840, Leases , the Company is deemed to be the owner of the leased space during the construction period because of certain indemnification provisions within the lease agreement. As a result, as of September 30, 2018, the Company capitalized $24.7 million (equal to the estimated fair value of its leased portion of the premises) as construction-in-progress within property, plant and equipment and recorded a corresponding build-to-suit facility lease financing obligation. As of September 30, 2018, the current portion of the lease financing obligation of $3.5 million was classified within accrued expenses and other current liabilities and the remaining $21.2 million was classified as a lease liability, net of current portion, on its consolidated balance sheet. The construction is expected to be completed in the fourth quarter of 2018, at which time the Company will assess and determine if the assets and corresponding liability should be de-recognized. Future Lease Payments As of September 30, 2018, minimum commitments under the Company’s facilities’ leases are as follows (in thousands): Year Ending December 31, 2018 (three months ending December 31) $ 984 2019 4,470 2020 4,604 2021 4,555 2022 4,128 Thereafter 16,995 $ 35,736 License Agreement with the Whitehead Institute for Biomedical Research The Company has a license agreement with the Whitehead Institute for Biomedical Research (“WIBR”), as amended, under which the Company has been granted an exclusive, sublicensable, nontransferable license under certain patent families related to the development of the Company’s red cell therapies (the “WIBR License”). The Company is obligated to pay WIBR annual license maintenance fees of less than $0.1 million as well as patent-related costs, including legal fees, and low single-digit royalties based on annual net sales of licensed products and licensed services by the Company and its sublicensees. Based on the progress the Company makes in the advancement of products covered by the licensed patent rights, the Company is required to make aggregate milestone payments of up to $1.6 million upon the achievement of specified preclinical, clinical and regulatory milestones. In addition, the Company is required to pay to WIBR a percentage of the non-royalty payments that it receives from sublicensees of the patent rights licensed by WIBR. This percentage varies from low single-digit to low double-digit percentages and will be based upon the clinical stage of the product that is the subject of the sublicense. Royalties shall be paid by the Company on a licensed product-by-licensed product and country-by-country basis, beginning on the first commercial sale of such licensed product in such country until expiration of the last valid patent claim covering such licensed product in such country. The Company has the right to terminate the WIBR License in its entirety, on a patent-by-patent or country-by-country basis, at will upon three months’ notice to WIBR. WIBR may terminate the agreement upon breach of contract or in the event of the Company’s bankruptcy, liquidation, insolvency or cessation of business related to the license. 401(k) Plan In January 2018, the Company established a defined-contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. The Company will make matching contributions at a rate of 50% of each employee’s contribution up to a maximum employee contribution of 6% of eligible plan compensation. For each of the three and nine months ended September 30, 2018, the Company made matching contributions of $0.1 million and $0.2 million, respectively. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, contract research organizations, 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 and its executive officers 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. The Company has not incurred any material costs as a result of such indemnifications and is not currently aware of any indemnification claims. Legal Proceedings The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2018
Net Loss Per Share
Net Loss Per Share	12. 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 September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss $ (26,362) $ (11,919) $ (62,012) $ (26,330) Accretion of Series A redeemable convertible preferred stock to redemption value — — — (656) Net loss attributable to common stockholders $ (26,362) $ (11,919) $ (62,012) $ (26,986) Denominator: Weighted average common shares outstanding, basic and diluted 62,311,111 8,078,196 26,662,233 7,899,507 Net loss per share attributable to common stockholders, basic and diluted $ (0.42) $ (1.48) $ (2.33) $ (3.42) The Company’s potential dilutive securities have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares from the periods in the table above, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: September 30, 2018 2017 Convertible preferred stock (as converted to common stock) — 43,933,006 Warrants to purchase convertible preferred stock (as converted to common stock) — 135,567 Restricted common stock 2,644,423 6,627,014 Stock options to purchase common stock 14,701,283 3,705,030 17,345,706 54,400,617

Related Parties

Related Parties	9 Months Ended
	Sep. 30, 2018
Related Parties
Related Parties	13. Related Parties In April 2013, the Company entered into a services agreement with Flagship Ventures Management, Inc. (“Flagship”), an affiliate of one of its principal stockholders, to provide general and administrative services to the Company, including certain consulting services and the provision of employee health and dental benefit plans for the Company’s employees. The Company recorded general and administrative expense for services received under this agreement of $0.3 million and $0.8 million during the three and nine months ended September 30, 2018, respectively. The Company made cash payments of $0.4 million and $0.8 million for the three and nine months ended September 30, 2018, respectively, related to these services. The Company recorded general and administrative expense for services received under this agreement of $0.2 million and $0.7 million during the three and nine months ended September 30, 2017, respectively. The Company made cash payments of $0.2 million and $0.7 million for the three and nine months ended September 30, 2017, respectively, related to these services. As of September 30, 2018, the Company had no amounts payable to Flagship for costs related to the services agreement. In January 2017, the Company loaned $0.7 million to the chairman of its board of directors to purchase shares of common stock pursuant to a promissory note and a restricted stock agreement (see Note 10). In May 2017, the Company loaned $1.8 million to the chairman of its board of directors to purchase shares of common stock pursuant to a promissory note and a restricted stock agreement (see Note 10). On June 21, 2018, the aggregate principal balance of both promissory notes of $2.5 million and all interest that had accrued thereon, totaling $0.1 million, was forgiven by the Company and the promissory notes were terminated (see Note 10). In April 2017, the Company loaned $0.2 million to an executive officer of the Company to purchase shares of common stock pursuant to two promissory notes and two restricted stock agreements (see Note 10). On June 21, 2018, the aggregate principal balance of both promissory notes of $0.2 million and all interest that had accrued thereon, totaling less than $0.1 million, was repaid in full by the executive officer and the promissory notes were terminated.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2018
Subsequent Events
Subsequent Events	14. Subsequent Events Lease Amendment In November 2018, the Company amended its January 2018 lease to lease additional office and laboratory space at 399 Binney Street in Cambridge, Massachusetts (the “Expansion Space”). The term for the Expansion Space is expected to commence in August 2019 and expires nine years from the commencement date. The initial annual base rent for the Expansion Space is approximately $2.5 million and such amount will increase by 3% annually on the anniversary of the commencement date. The Company is obligated to pay its portion of real estate taxes and costs related to the Expansion Space, including costs of operations, maintenance, repair, replacement and property management. In connection with the lease amendment, the Company increased the letter of credit held for the benefit of the landlord by $0.6 million, which is secured by a cash deposit of the same amount.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Summary of Significant Accounting Policies
Unaudited Interim Financial Information	Unaudited Interim Financial Information The consolidated balance sheet at December 31, 2017 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited consolidated financial statements as of September 30, 2018 and for the three and nine months ended September 30, 2018 and 2017 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These 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, 2017 included in the Company’s Registration Statement on Form S-1, as amended, File No. 333-225840 on file with the SEC. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s consolidated financial position as of September 30, 2018 and consolidated results of operations for the three and nine months ended September 30, 2018 and 2017 and the consolidated cash flows for the nine months ended September 30, 2018 and 2017, have been made. The Company’s consolidated results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the valuation of common stock prior to the IPO, the valuation of stock-based awards and the valuation of the preferred stock warrant liability prior to the IPO. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions.
Concentrations of Credit Risk and of Significant Suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. As of September 30, 2018 and December 31, 2017, the Company did not maintain cash balances in excess of federally insured limits. The Company’s cash equivalents, as of September 30, 2018, consisted of U.S. government money market funds, U.S. government agency bonds and U.S. government treasury notes. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and raw materials for its development programs. These programs could be adversely affected by a significant interruption in these manufacturing services or the availability of raw materials.
Restricted Cash	Restricted Cash As of September 30, 2018 and December 31, 2017, the Company maintained letters of credit totaling $1.2 million and $0.3 million, respectively, for the benefit of the landlords of its leased properties. The Company was required to maintain separate cash balances of these amounts to secure the letters of credit. Related to these separate cash balances, the Company classified $1.1 million and $0.3 million as restricted cash (non-current) in its consolidated balance sheet as of September 30, 2018 and December 31, 2017, respectively, and classified $0.1 million as restricted cash (current) in its consolidated balance sheet as of September 30, 2018.
Cash Equivalents	Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.
Investments	Investments The Company’s investments are classified as available-for-sale and are carried at fair value, with the unrealized gains and losses reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value determined to be other than temporary are based on the specific identification method and are included as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company classifies its investments with maturities beyond one year as short-term, based on their highly liquid nature and because such investments are available for current operations. The Company evaluates its investments with unrealized losses for other-than-temporary impairment. When assessing investments for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: · Level 1—Quoted prices in active markets for identical assets or liabilities. · Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. · Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s long-term debt approximates its fair value due to its variable interest rate, which approximates a market interest rate.
Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board, (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Compensation — Stock Compensation (Topic 718), Improvements to Non-Employee Share-Based Payment Accounting (“ASU 2018-07”) . This ASU is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. For public entities, ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For non-public entities, ASU 2018-07 is effective for annual periods beginning after December 15, 2019. Early adoption is permitted for all entities but no earlier than the Company’s adoption of ASU 2014-09. The Company adopted ASU 2018-07 effective January 1, 2018 by remeasuring outstanding equity-classified awards issued to non-employees for which a measurement date had not been established as of January 1, 2018 through a cumulative-effect adjustment to accumulated deficit as of January 1, 2018. The Company has elected to estimate the expected term of options utilizing the simplified method for both employee and non-employee options that qualify as plain-vanilla options. The Company has elected to account for forfeitures for non-employee options as they occur rather than apply an estimated forfeiture rate to stock-based compensation expense . The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of ASU 2018-07 on January 1, 2018 (in thousands): Balance at Balance at December 31, 2017 Adjustments January 1, 2018 Additional paid-in capital $ 17,277 $ (92) $ 17,185 Accumulated deficit $ (60,979) $ 92 $ (60,887) The $0.1 million adjustment is the result of the change in fair value of the unvested awards, representing awards for which a measurement date had not been established, using an expected term rather than the contractual term of the awards. As a result of adopting ASU 2018-07, the results for the three months ended March 31, 2018 and three and six months ended June 30, 2018 have been revised to reflect the adoption of ASU 2018-07 effective January 1, 2018. These revisions will be reflected in future filings containing such financial information. The following tables summarize the impact of adoption to the Company’s previously issued consolidated balance sheets and consolidated statements of operations and comprehensive loss (in thousands): Consolidated Balance Sheets As of March 31, 2018 As of June 30, 2018 As previously As previously reported As revised reported As revised Additional paid-in capital $ 20,738 $ 19,802 $ 34,636 $ 25,840 Accumulated deficit $ (76,234) $ (75,298) $ (105,333) $ (96,537) Consolidated Statements of Operations and Comprehensive Loss Three Months Ended March 31, 2018 Three Months Ended June 30, 2018 Six Months Ended June 30, 2018 As previously As previously As previously reported As revised reported As revised reported As revised Operating expenses: Research and development $ 9,650 $ 9,506 $ 11,965 $ 11,361 $ 21,615 $ 20,867 General and administrative 5,797 5,097 16,279 9,023 22,076 14,120 Total operating expenses 15,447 14,603 28,244 20,384 43,691 34,987 Loss from operations (15,447) (14,603) (28,244) (20,384) (43,691) (34,987) Net loss attributable to common stockholders $ (15,255) $ (14,411) $ (29,099) $ (21,239) $ (44,354) $ (35,650) Net loss per share attributable to common stockholders, basic and diluted $ (1.83) $ (1.72) $ (3.33) $ (2.43) $ (5.19) $ (4.17) Comprehensive loss: Net loss (15,255) (14,411) (29,099) (21,239) (44,354) (35,650) Other comprehensive income (loss): Unrealized gains (losses) on investments, net of tax of $0 (45) (45) 18 18 (27) (27) Comprehensive loss $ (15,300) $ (14,456) $ (29,081) $ (21,221) $ (44,381) $ (35,677) As of the adoption date of January 1, 2018, the Company had 330,917 outstanding options to non-employees for which a measurement date had not been established. As of the adoption date of January 1, 2018, the Company had 4,767,014 shares of restricted common stock held by non-employees that were being accounted for as stock options for which a measurement date had not been established (see Note 10). The weighted average fair value of these awards was $5.88 per share as of January 1, 2018. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of outstanding awards granted to non-employees for which a measurement date had not been established as of the adoption date of January 1, 2018: Risk-free interest rate 2.3 % Expected volatility 74 % Expected dividend yield — Expected term (in years) 6.1 Common stock value $ 6.28 Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. For public entities, the guidance is effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. For nonpublic entities, the guidance is effective for annual reporting periods beginning after December 15, 2019 and for interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted for all entities. ASU 2016-02 initially required adoption using a modified retrospective approach, under which all years presented in the financial statements would be prepared under the revised guidance. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842) , which added an optional transition method under which financial statements may be prepared under the revised guidance for the year of adoption, but not for prior years. Under the latter method, entities will recognize a cumulative catch-up adjustment to the opening balance of retained earnings in the period of adoption. The Company is currently evaluating whether to early-adopt ASU 2016-02 and evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements. In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception (“ASU 2017-11”). Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within Accounting Standards Codification (“ASC”) Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. For public entities, this guidance is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For nonpublic entities, this guidance is effective for annual periods beginning after December 15, 2019 and for interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted for all entities, including adoption in an interim period. If an entity early adopts the amendments in an interim period, any adjustments should be reflected as of the beginning of the fiscal year that includes that interim period. The Company is currently evaluating whether to early-adopt ASU 2017-11 and evaluating the impact that the adoption of ASU 2017-11 will have on its consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle
Summary of cumulative effect of adopting ASC 2018-07 using the modified retrospective method	The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of ASU 2018-07 on January 1, 2018 (in thousands): Balance at Balance at December 31, 2017 Adjustments January 1, 2018 Additional paid-in capital $ 17,277 $ (92) $ 17,185 Accumulated deficit $ (60,979) $ 92 $ (60,887)
Non-employees
New Accounting Pronouncements or Change in Accounting Principle
Schedule of assumptions used to determine the fair value of stock awards granted	Risk-free interest rate 2.3 % Expected volatility 74 % Expected dividend yield — Expected term (in years) 6.1 Common stock value $ 6.28
ASU 2018-07
New Accounting Pronouncements or Change in Accounting Principle
Summary of impact of accounting standard adoption	The following tables summarize the impact of adoption to the Company’s previously issued consolidated balance sheets and consolidated statements of operations and comprehensive loss (in thousands): Consolidated Balance Sheets As of March 31, 2018 As of June 30, 2018 As previously As previously reported As revised reported As revised Additional paid-in capital $ 20,738 $ 19,802 $ 34,636 $ 25,840 Accumulated deficit $ (76,234) $ (75,298) $ (105,333) $ (96,537) Consolidated Statements of Operations and Comprehensive Loss Three Months Ended March 31, 2018 Three Months Ended June 30, 2018 Six Months Ended June 30, 2018 As previously As previously As previously reported As revised reported As revised reported As revised Operating expenses: Research and development $ 9,650 $ 9,506 $ 11,965 $ 11,361 $ 21,615 $ 20,867 General and administrative 5,797 5,097 16,279 9,023 22,076 14,120 Total operating expenses 15,447 14,603 28,244 20,384 43,691 34,987 Loss from operations (15,447) (14,603) (28,244) (20,384) (43,691) (34,987) Net loss attributable to common stockholders $ (15,255) $ (14,411) $ (29,099) $ (21,239) $ (44,354) $ (35,650) Net loss per share attributable to common stockholders, basic and diluted $ (1.83) $ (1.72) $ (3.33) $ (2.43) $ (5.19) $ (4.17) Comprehensive loss: Net loss (15,255) (14,411) (29,099) (21,239) (44,354) (35,650) Other comprehensive income (loss): Unrealized gains (losses) on investments, net of tax of $0 (45) (45) 18 18 (27) (27) Comprehensive loss $ (15,300) $ (14,456) $ (29,081) $ (21,221) $ (44,381) $ (35,677)

Investments and Fair Value of_2

Investments and Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2018
Investments and Fair Value of Financial Assets and Liabilities
Schedule of investments by security type	Investments by security type consisted of the following (in thousands): September 30, 2018 Gross Gross Unrealized Unrealized Amortized Cost Gains Losses Fair Value U.S. treasury notes (due within one year) $ 41,154 $ — $ (15) $ 41,139 U.S. government agency bonds (due within one year) 10,453 — (4) 10,449 $ 51,607 $ — $ (19) $ 51,588
Schedule of financial assets and liabilities measured at fair value on a recurring basis	The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements at September 30, 2018 Using: Not Subject to Level 1 Level 2 Level 3 Leveling Total Assets: Cash equivalents: Money market funds $ 354,786 $ — $ — $ — $ 354,786 Investments: U.S. government agency bonds — 10,449 — — 10,449 U.S. treasury notes — 41,139 — — 41,139 Receivable from unsettled maturity of a security (1) — — — $ 2,500 2,500 $ 354,786 $ 51,588 $ — $ 2,500 $ 408,874 (1) Represents a receivable from the maturity of a security, which ocurred during the three months ended September 30, 2018 and for which cash settlement was received in October 2018. Fair Value Measurements at December 31, 2017 Using: Not Subject to Level 1 Level 2 Level 3 Leveling Total Assets: Cash equivalents: Money market funds $ 104,288 $ — $ — $ — $ 104,288 Liabilities: Preferred stock warrant liability $ — $ — $ 866 $ — $ 866
Roll-forward of aggregate fair values of preferred stock warrants for which fair value determined by Level 3 inputs	The following table provides a roll-forward of the aggregate fair values of the Company’s preferred stock warrants for which fair value was determined by Level 3 inputs (in thousands): Preferred stock warrant liability Fair value at December 31, 2017 $ 866 Change in fair value through the exercise date 2,187 Reclassification to additional paid-in capital in connection with IPO (3,053) Fair value at September 30, 2018 $ —

Property, Plant and Equipment_2

Property, Plant and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2018
Property, Plant and Equipment, Net.
Schedule of property, plant and equipment, Net	Property, plant and equipment, net consisted of the following (in thousands): September 30, 2018 December 31, 2017 Laboratory equipment $ 6,151 $ 2,751 Land 1,300 — Leasehold improvements 117 117 Computer equipment 139 57 Construction-in-progress 32,964 74 40,671 2,999 Less: Accumulated depreciation and amortization (1,463) (584) $ 39,208 $ 2,415

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2018
Accrued Expenses and Other Current Liabilities
Schedule of accrued expenses and other current liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): September 30, 2018 December 31, 2017 Accrued employee compensation and benefits $ 1,977 $ 1,339 Accrued external research and development expenses 2,171 1,230 Accrued lease liability, current portion 3,451 — Accrued professional fees 990 219 Accrued construction-in-progress 1,336 — Other 320 198 $ 10,245 $ 2,986

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2018
Debt
Schedule of estimated future principal payments due	As of September 30, 2018, the estimated future principal payments due were as follows (in thousands): Year Ending December 31, 2018 (three months ending December 31) $ — 2019 2,139 2020 3,361 $ 5,500

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Stock-Based Compensation
Schedule of allocation of share based compensation	The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development expenses $ 1,347 $ 170 $ 2,331 $ 1,390 General and administrative expenses 8,540 3,967 16,071 7,417 $ 9,887 $ 4,137 $ 18,402 $ 8,807
Employees, directors and non-employees
Stock-Based Compensation
Schedule of assumptions used to determine the fair value of stock awards granted	Three Months Ended September 30, Nine Months Ended September 30, 2018 2018 Risk-free interest rate 2.82 % 2.70 % Expected volatility 74 % 74 % Expected dividend yield — — Expected term (in years) 6.4 6.2

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies
Schedule of minimum commitments under the Companys facilities lease	As of September 30, 2018, minimum commitments under the Company’s facilities’ leases are as follows (in thousands): Year Ending December 31, 2018 (three months ending December 31) $ 984 2019 4,470 2020 4,604 2021 4,555 2022 4,128 Thereafter 16,995 $ 35,736

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2018
Net Loss Per Share
Schedule of calculation 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 September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss $ (26,362) $ (11,919) $ (62,012) $ (26,330) Accretion of Series A redeemable convertible preferred stock to redemption value — — — (656) Net loss attributable to common stockholders $ (26,362) $ (11,919) $ (62,012) $ (26,986) Denominator: Weighted average common shares outstanding, basic and diluted 62,311,111 8,078,196 26,662,233 7,899,507 Net loss per share attributable to common stockholders, basic and diluted $ (0.42) $ (1.48) $ (2.33) $ (3.42)
Schedule of dilutive securities excluded from computations of diluted weighted average shares outstanding	September 30, 2018 2017 Convertible preferred stock (as converted to common stock) — 43,933,006 Warrants to purchase convertible preferred stock (as converted to common stock) — 135,567 Restricted common stock 2,644,423 6,627,014 Stock options to purchase common stock 14,701,283 3,705,030 17,345,706 54,400,617

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation (Details) - USD ($) $ in Thousands	Jul. 20, 2018	Jul. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Sale of Equity
Net proceeds								$ 257,866
Recurring losses			$ (26,362)	$ (21,239)	$ (14,411)	$ (11,919)	$ (35,650)	(62,012)	$ (26,986)	$ 43,800
Accumulated deficit			$ (122,899)					$ (122,899)		$ (60,979)
Common stock
Sale of Equity
Number of common stock issued on conversion	51,845,438
IPO
Sale of Equity
Shares issued (in shares)	12,055,450
Share exercised through underwriters (in shares)	1,572,450
Net proceeds	$ 254,300
Number of common stock issued on conversion		131,273

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies
Restricted cash for the benefit of the landlords of leased properties	$ 1,200	$ 300
Restricted cash (non-current)	1,102	$ 284
Restricted cash (current)	$ 122

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($)	Jan. 01, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Additional paid-in capital		$ 533,846,000					$ 533,846,000		$ 17,277,000
Accumulated deficit		(122,899,000)					(122,899,000)		(60,979,000)
Change in the fair value of the unvested awards	$ 100,000
Operating expenses:
Research and development		14,363,000	$ 11,361,000	$ 9,506,000	$ 6,126,000	$ 20,867,000	35,230,000	$ 14,628,000
General and administrative		13,191,000	9,023,000	5,097,000	5,849,000	14,120,000	27,311,000	11,163,000
Total operating expenses		27,554,000	20,384,000	14,603,000	11,975,000	34,987,000	62,541,000	25,791,000
Loss from operations		(27,554,000)	(20,384,000)	(14,603,000)	(11,975,000)	(34,987,000)	(62,541,000)	(25,791,000)
Net loss attributable to common stockholders		$ (26,362,000)	$ (21,239,000)	$ (14,411,000)	$ (11,919,000)	$ (35,650,000)	$ (62,012,000)	$ (26,986,000)	$ 43,800,000
Net loss per share attributable to common stockholders, basic and diluted		$ (0.42)	$ (2.43)	$ (1.72)	$ (1.48)	$ (4.17)	$ (2.33)	$ (3.42)
Comprehensive loss:
Net loss		$ (26,362,000)	$ (21,239,000)	$ (14,411,000)	$ (11,919,000)	$ (35,650,000)	$ (62,012,000)	$ (26,330,000)
Other comprehensive income (loss):
Unrealized gains (losses) on investments, net of tax of $0		8,000	18,000	(45,000)		(27,000)	(19,000)
Comprehensive loss		(26,354,000)	(21,221,000)	(14,456,000)	(11,919,000)	(35,677,000)	(62,031,000)	(26,330,000)
Unrealized gains (losses) on investments, tax		$ 0	0	0	$ 0	0	$ 0	$ 0
Assumptions used to determine the fair value of outstanding awards granted
Weighted average grant-date fair value (per share)							$ 8.64
Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Weighted average grant-date fair value (per share)	$ 5.88
Risk-free interest rate	2.30%
Expected volatility	74.00%
Expected term (in years)	6 years 1 month 6 days
Common stock value	$ 6.28
Stock options | Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Number of outstanding options	330,917
Restricted Common Stock | Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Number of outstanding options	4,767,014
As previously reported
Operating expenses:
Research and development			11,965,000	9,650,000		21,615,000
General and administrative			16,279,000	5,797,000		22,076,000
Total operating expenses			28,244,000	15,447,000		43,691,000
Loss from operations			(28,244,000)	(15,447,000)		(43,691,000)
Net loss attributable to common stockholders			$ (29,099,000)	$ (15,255,000)		$ (44,354,000)
Net loss per share attributable to common stockholders, basic and diluted			$ (3.33)	$ (1.83)		$ (5.19)
Comprehensive loss:
Net loss			$ (29,099,000)	$ (15,255,000)		$ (44,354,000)
Other comprehensive income (loss):
Unrealized gains (losses) on investments, net of tax of $0			18,000	(45,000)		(27,000)
Comprehensive loss			(29,081,000)	(15,300,000)		(44,381,000)
ASU 2018-07
Additional paid-in capital	$ 17,185,000		25,840,000	19,802,000		25,840,000
Accumulated deficit	(60,887,000)		(96,537,000)	(75,298,000)		(96,537,000)
ASU 2018-07 | As previously reported
Additional paid-in capital			34,636,000	20,738,000		34,636,000
Accumulated deficit			$ (105,333,000)	$ (76,234,000)		$ (105,333,000)
ASU 2018-07 | Adjustments
Additional paid-in capital	(92,000)
Accumulated deficit	$ 92,000

Investments and Fair Value of_3

Investments and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Marketable securities
Amortized Cost	$ 51,607
Gross Unrealized Losses	(19)
Fair Value	51,588
Assets:
Investments	51,588
Transfers between Level 1, Level 2 or Level 3
Transfer from Level 1 to Level 2, assets	0
Transfer from Level 2 to Level 1, assets	0
Transfers into Level 3, assets	0
Transfer out of Level 3, assets	0
Transfer from Level 1 to Level 2, liabilities	0
Transfer from Level 2 to Level 1, liabilities	0
Transfers into Level 3, liabilities	0
Transfer out of Level 3, liabilities	0
U.S. government agency bonds
Marketable securities
Amortized Cost	10,453
Gross Unrealized Losses	(4)
Fair Value	10,449
Assets:
Investments	10,449
U.S. treasury notes
Marketable securities
Amortized Cost	41,154
Gross Unrealized Losses	(15)
Fair Value	41,139
Assets:
Investments	41,139
Preferred stock warrant liability
Roll forward of aggregate fair values of preferred stock warrants
Fair value at beginning of period	866
Change in fair value through the exercise date	2,187
Reclassification to additional paid-in capital in connection with IPO	(3,053)
Recurring
Assets:
Assets	408,874
Recurring | Money market funds
Assets:
Cash equivalents	354,786	$ 104,288
Recurring | U.S. government agency bonds
Marketable securities
Fair Value	10,449
Assets:
Investments	10,449
Recurring | U.S. treasury notes
Marketable securities
Fair Value	41,139
Assets:
Investments	41,139
Recurring | Receivable from unsettled maturity of a security
Marketable securities
Fair Value	2,500
Assets:
Investments	2,500
Recurring | Preferred stock warrant liability
Liabilities:
Liabilities		866
Recurring | Level 1
Assets:
Assets	354,786
Recurring | Level 1 | Money market funds
Assets:
Cash equivalents	354,786	104,288
Recurring | Level 2
Assets:
Assets	51,588
Recurring | Level 2 | U.S. government agency bonds
Marketable securities
Fair Value	10,449
Assets:
Investments	10,449
Recurring | Level 2 | U.S. treasury notes
Marketable securities
Fair Value	41,139
Assets:
Investments	41,139
Recurring | Level 3 | Preferred stock warrant liability
Liabilities:
Liabilities		$ 866
Recurring | Not Subject to Leveling
Assets:
Assets	2,500
Recurring | Not Subject to Leveling | Receivable from unsettled maturity of a security
Marketable securities
Fair Value	2,500
Assets:
Investments	$ 2,500

Property, Plant and Equipment_3

Property, Plant and Equipment, Net (Details) $ in Thousands	Jul. 31, 2018USD ($)ft²	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		$ 40,671	$ 2,999
Less: Accumulated depreciation and amortization		(1,463)	(584)
Property, plant and equipment, net		39,208	2,415
Laboratory equipment
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		6,151	2,751
Land
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		1,300
Purchases and additions	$ 1,300
Leasehold improvements
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		117	117
Construction-in-progress		500
Computer equipment
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		139	57
Construction-in-progress
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		32,964	$ 74
Purchases and additions	$ 6,700
Build-to-suit lease
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		24,700
Manufacturing facility
Property, Plant and Equipment, Net
Square footing of facility acquired | ft²	135,000
Purchases and additions	$ 8,000
Technology Service | Construction-in-progress
Property, Plant and Equipment, Net
Purchases and additions		$ 1,000

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accrued Expenses and Other Current Liabilities
Accrued employee compensation and benefits	$ 1,977	$ 1,339
Accrued external research and development expenses	2,171	1,230
Accrued lease liability, current portion	3,451
Accrued professional fees	990	219
Accrued construction-in-progress	1,336
Other	320	198
Accrued expenses and other current liabilities	$ 10,245	$ 2,986

Debt (Details)

Debt (Details) - USD ($) $ in Thousands	1 Months Ended
	May 31, 2018	May 31, 2017	Sep. 30, 2018
Estimated future principal payments due
2,019			$ 2,139
2,020			3,361
Total debt			5,500
2015 Credit Facility
Debt
Principal amount			$ 5,500
Floor rate	5.50%	4.50%
Percentage of borrowing interest rate			6.00%
Percentage of change in debt modification from debt extinguishment	10.00%
2015 Credit Facility | Prime rate
Debt
Variable interest rate	0.75%	1.25%

Convertible Preferred Stock (De

Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		9 Months Ended
	Feb. 28, 2018	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Convertible Preferred Stock
Proceeds from issuance of convertible preferred stock			$ 100,986	$ 120,117
Issuance cost			$ 3,449
Preferred stock carrying value					$ 139,790
Convertible preferred stock
Convertible Preferred Stock
Preferred stock issued					43,933,006
Preferred stock authorized					44,070,808
Preferred stock outstanding					43,933,006
Preferred stock, liquidation preference					$ 138,242
Series A Preferred Stock
Convertible Preferred Stock
Percentage of cumulative dividends per year			8.00%
Convertible preferred stock redemption period			5 years
Maximum percentage of change in fair value of preferred stock			10.00%
Series B Preferred Stock
Convertible Preferred Stock
Preferred stock issued		14,362,344
Convertible preferred stock issue price (dollars per share)		$ 8.39
Proceeds from issuance of convertible preferred stock		$ 120,500
Issuance cost		$ 400
Series C Preferred Stock
Convertible Preferred Stock
Preferred stock issued	7,912,432
Convertible preferred stock issue price (dollars per share)	$ 12.79
Proceeds from issuance of convertible preferred stock	$ 101,200
Issuance cost	$ 200

Warrants to Purchase Converti_2

Warrants to Purchase Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	Jul. 31, 2018	May 31, 2017	Dec. 31, 2015
Warrants to purchase convertible preferred stock
Maximum number of preferred shares can purchase from outstanding warrants	135,567
IPO
Warrants to purchase convertible preferred stock
Warrants to purchase aggregate	135,567
Issuance of warrants exercise	131,273
Common stock exercise price	4,294
Series A Preferred Stock
Warrants to purchase convertible preferred stock
Maximum number of preferred shares can purchase from outstanding warrants			133,333
Warrants exercisable price			$ 0.60
Warrants contractual term (in years)			10 years
Series B Preferred Stock
Warrants to purchase convertible preferred stock
Maximum number of preferred shares can purchase from outstanding warrants		2,234
Warrants exercisable price		$ 8.39
Warrants contractual term (in years)		10 years
Series B Preferred Stock | Maximum
Warrants to purchase convertible preferred stock
Fair value of warrants		$ 0.1

Equity (Details)

Equity (Details) - $ / shares	1 Months Ended
	Jul. 31, 2018	Sep. 30, 2018	Jul. 20, 2018	Jun. 30, 2018	Apr. 30, 2018	Feb. 28, 2018	Jan. 31, 2018	Dec. 31, 2017
Equity
Common stock, authorized (in shares)		150,000,000	150,000,000	79,000,000	78,800,000	75,000,000	65,000,000	65,000,000
Shares authorized			160,000,000
Common stock, par value (in dollars per share)		$ 0.001	$ 0.001					$ 0.001
Preferred stock, share authorized (in shares)		10,000,000	10,000,000
Preferred stock, par value (in dollars per share)		$ 0.001	$ 0.001
Maximum number of preferred shares can purchase from outstanding warrants	135,567
Common stock shares issued for warrants	131,273
Common stock shares withheld	4,294

Stock-Based Compensation (Detai

Stock-Based Compensation (Details) - shares	Jul. 06, 2018	Sep. 30, 2018
2014 Stock Incentive Plan
Stock-Based Compensation
Total number of shares of common stock authorized to issue		19,152,328
Number of shares remained available for future issuance		47,447
2018 Equity Incentive Plan
Stock-Based Compensation
Total number of shares of common stock authorized to issue	5,708,931
Annual increase of shares reserved for awards (as a percent)	4.00%
2018 Employee Stock Purchase Plan
Stock-Based Compensation
Total number of shares of common stock authorized to issue	951,488
Annual increase of shares reserved for awards (as a percent)	1.00%
Annual increase of shares reserved for awards if less than 1% of the outstanding stock	951,488

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Option Valuation (Details) - Employees, directors and non-employees	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Stock-Based Compensation
Risk-free interest rate	2.82%	2.70%
Expected volatility	74.00%	74.00%
Expected term (in years)	6 years 4 months 24 days	6 years 2 months 12 days

Stock-Based Compensation - Perf

Stock-Based Compensation - Performance based stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 21, 2018	Apr. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock-Based Compensation
Options granted					10,122,023
Weighted average exercise price					$ 11.98
Weighted average grant-date fair value (per share)					$ 8.64
Proceeds from issuance of common stock upon exercise of stock options					$ 18	$ 29
Receivables from promissory note recorded in balance sheet		$ 0
Stock-based compensation expense			$ 9,887	$ 4,137	18,402	$ 8,807
Executive officer
Stock-Based Compensation
Number of options exercised		1,400,000
Options exercise price		$ 0.18
Proceeds from issuance of common stock upon exercise of stock options		$ 100
Amount of promissory note received from early exercise of stock options		200
Principal balance of promissory note repaid	$ 200
Interest on promissory note repaid	$ 100
Stock options
Stock-Based Compensation
Liability from early exercise of stock options		$ 100
Performance-based stock Options
Stock-Based Compensation
Stock-based compensation expense			$ 2,100		$ 2,100
Performance-based stock Options | Performance-based vesting
Stock-Based Compensation
Options that vested			447,000
Options conditioned not met			232,500		232,500

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Common Stock (Details) $ / shares in Units, $ in Thousands	Jun. 21, 2018USD ($)	May 31, 2017USD ($)item$ / sharesshares	Apr. 30, 2017USD ($)$ / sharesshares	Jan. 31, 2017USD ($)$ / sharesshares	Sep. 30, 2018USD ($)	Sep. 30, 2018USD ($)
Executive officer
Stock-Based Compensation
Number of options exercised | shares			1,400,000
Restricted stock exercise price | $ / shares			$ 0.18
Principal balance of promissory note repaid	$ 200
Interest on promissory note repaid	100
Restricted Common Stock
Stock-Based Compensation
Amount recorded in balance sheet due to promissory notes received from sale of common stock		$ 0	$ 0	$ 0
Principal amount of promissory note forgiven	2,500
Interest on promissory note forgiven	100
Incremental share based compensation expense					$ 200	$ 1,400
Restricted Common Stock | Executive officer
Stock-Based Compensation
Number of shares sold | shares			460,000
Restricted stock exercise price | $ / shares			$ 0.19
Principal amount of promissory note received in exchange for restricted common stock			$ 100
Principal balance of promissory note repaid	100
Interest on promissory note repaid	$ 100
Restricted Common Stock | Board of Directors Chairman
Stock-Based Compensation
Number of shares sold | shares		1,100,000		3,667,014
Restricted stock exercise price | $ / shares		$ 1.65		$ 0.19
Principal amount of promissory note received in exchange for restricted common stock		$ 2,500
Number of promissory notes | item		2

Stock-Based Compensation - Comp

Stock-Based Compensation - Compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock-based compensation expense
Stock-based compensation expense	$ 9,887	$ 4,137	$ 18,402	$ 8,807
Research and development expenses
Stock-based compensation expense
Stock-based compensation expense	1,347	170	2,331	1,390
General and administrative expenses
Stock-based compensation expense
Stock-based compensation expense	8,540	$ 3,967	16,071	$ 7,417
Employees, directors and non-employees
Stock-based compensation expense
Unrecognized compensation cost	$ 93,400		$ 93,400
Unrecognized compensation cost expected to be recognized over a weighted average period			3 years

Commitments and Contingencies -

Commitments and Contingencies - Operating Lease (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)item	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)item	Sep. 30, 2017USD ($)
Commitments and Contingencies
Number of leases | item	2		2
Rent expense | $	$ 0.4	$ 0.3	$ 1.1	$ 0.6

Commitments and Contingencies_2

Commitments and Contingencies - Capital Lease (Details) - USD ($) $ in Thousands	1 Months Ended
	Jan. 31, 2018	Sep. 30, 2018
Commitments and Contingencies
Lease term	8 years
Renewal option term	5 years
Initial base rent	$ 3,800
Base rent incremental (as a percentage)	3.00%
Benefit of the landlord secured by cash deposit	$ 900
Tenant improvement allowance	$ 9,400
Construction-in-progress within property and equipment		$ 24,700
Current portion of the lease financing obligation		3,500
Lease liability, net of current portion		21,206
Minimum commitments under this lease
2018 (three months ending December 31)		984
2,019		4,470
2,020		4,604
2,021		4,555
2,022		4,128
Thereafter		16,995
Total		$ 35,736

Commitments and Contingencies_3

Commitments and Contingencies - Collaborative Arrangements And Non-collaborative Arrangement (Details) - WIBR $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Collaborative Arrangements and Non-collaborative Arrangement
Aggregate milestone payments	$ 1.6
Maximum | License maintenance fees
Collaborative Arrangements and Non-collaborative Arrangement
Licence costs	$ 0.1

Commitments and Contingencies_4

Commitments and Contingencies - Defined Contribution Plan (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Jan. 31, 2018	Sep. 30, 2018	Sep. 30, 2018
Commitments and Contingencies
Employer matching contribution (as a percent)	50.00%
Maximum employee contribution	6.00%
Company matching contribution		$ 0.1	$ 0.2

Net Loss Per Share - Weighted A

Net Loss Per Share - Weighted Average Shares (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Numerator:
Net loss	$ (26,362)	$ (21,239)	$ (14,411)	$ (11,919)	$ (35,650)	$ (62,012)	$ (26,330)
Accretion of Series A redeemable convertible preferred stock to redemption value							(656)
Net loss attributable to common stockholders	$ (26,362)	$ (21,239)	$ (14,411)	$ (11,919)	$ (35,650)	$ (62,012)	$ (26,986)	$ 43,800
Denominator:
Weighted average common shares outstanding basic and diluted	62,311,111			8,078,196		26,662,233	7,899,507
Net loss per share attributable to common stockholders, basic and diluted	$ (0.42)	$ (2.43)	$ (1.72)	$ (1.48)	$ (4.17)	$ (2.33)	$ (3.42)

Net Loss Per Share - Antidiluti

Net Loss Per Share - Antidilutive Securities (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	17,345,706	54,400,617
Convertible preferred stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding		43,933,006
Warrants to purchase convertible preferred stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding		135,567
Restricted Common Stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	2,644,423	6,627,014
Stock options
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	14,701,283	3,705,030

Related Parties (Details)

Related Parties (Details) $ in Millions	Jun. 21, 2017USD ($)	May 31, 2017USD ($)	Apr. 30, 2017USD ($)item	Jan. 31, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)
Flagship | Services agreement
Related Parties
Payment of general and administrative expenses					$ 0.3	$ 0.2	$ 0.8	$ 0.7
Cash payments					$ 0.4	$ 0.2	$ 0.8	$ 0.7
Board of Directors Chairman
Related Parties
Amount loaned or remitted to related parties		$ 1.8		$ 0.7
Principal amount of promissory note forgiven	$ 2.5
Interest on promissory note forgiven	0.1
Executive officer
Related Parties
Amount loaned or remitted to related parties			$ 0.2
Number of promissory notes | item			2
Number of restricted stock agreements | item			2
Aggregate principal balance	0.2
Interest income form related parties	$ 0.1

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent events - Expansion Space - Office and laboratory space $ in Millions	1 Months Ended
	Nov. 30, 2018USD ($)
Subsequent Events
Lease term (in years)	9 years
Initial annual base rent	$ 2.5
Annual increase in base rent (as a percent)	3.00%
Increase in letter of credit held for the benefit of the landlord	$ 0.6
Letter of credit secured by cash deposit	$ 0.6