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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2019	Aug. 14, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	--12-31
Entity File Number	001-38938
Entity Registrant Name	Stoke Therapeutics, Inc.
Entity Central Index Key	0001623526
Entity Incorporation, State or Country Name	Delaware
Entity Tax Identification Number	471144582
Entity Address, Address Line One	45 Wiggins Ave
Entity Address, City or Town	Bedford
Entity Address, State or Province	Massachusetts
Entity Address, Postal Zip Code	01730
City Area Code	781
Local Phone Number	430-8200
Trading Symbol	STOK
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		32,718,134

Condensed consolidated balance sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 242,680	$ 105,399
Prepaid expenses and other current assets	1,505	548
Interest receivable	260	196
Total current assets	244,445	106,143
Restricted cash	205	204
Property and equipment, net	1,722	1,192
Total assets	246,372	107,539
Current liabilities:
Accounts payable	1,238	1,071
Accrued and other current liabilities	3,580	1,396
Total current liabilities	4,818	2,467
Long term liabilities	18	4
Total liabilities	4,836	2,471
Commitments and contingencies (Note 6)
Stockholders’ equity
Preferred Stock, value
Common stock, par value of $0.0001 per share; 300,000,000 shares authorized, 32,674,565 and 727,413 shares issued and outstanding as of June 30, 2019 and December 31, 2018, respectively	3
Additional paid-in capital	280,804	130,776
Accumulated deficit	(39,271)	(25,710)
Total stockholders’ equity	241,536	105,068
Total liabilities and stockholders’ equity	$ 246,372	107,539
Convertible Preferred Stock
Stockholders’ equity
Preferred Stock, value		2
Total stockholders’ equity		$ 2

Condensed consolidated balance sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0
Preferred stock, shares outstanding	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	32,674,565	727,413
Common stock, shares outstanding	32,674,565	727,413
Convertible Preferred Stock
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	225,584,874
Preferred stock, shares issued	0	22,677,585
Preferred stock, shares outstanding	0	22,677,585
Preferred stock, aggregate liquidation preference		$ 130,850

Condensed consolidated statements of operations and comprehensive loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses:
Research and development	$ 6,023	$ 1,888	$ 10,156	$ 3,140
General and administrative	2,422	1,122	4,611	1,782
Total operating expenses	8,445	3,010	14,767	4,922
Loss from operations	(8,445)	(3,010)	(14,767)	(4,922)
Other income (expense):
Interest income	629		1,210
Other expense, net	(3)		(4)
Total other income (expense)	626		1,206
Net loss	$ (7,819)	$ (3,010)	$ (13,561)	$ (4,922)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.54)	$ (4.36)	$ (4.57)	$ (7.14)
Weighted-average common shares outstanding, basic and diluted	5,083,620	691,212	2,970,292	689,112

Condensed consolidated statements of stockholders' equity - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Balance at Dec. 31, 2017	$ (1,292)			$ 11,897	$ (13,189)
Balance, Shares at Dec. 31, 2017		4,980,168	670,090
Net loss	(1,912)				(1,912)
Stock-based compensation	4			4
Issuance of common stock upon exercise of stock options	8			8
Issuance of common stock upon exercise of stock options, Shares			21,119
Issuance of convertible Preferred Stock, net of issuance costs of $88	15,412	$ 1		15,411
Issuance of convertible Preferred Stock, net of issuance costs of $88, Shares		4,071,554
Balance at Mar. 31, 2018	12,220	$ 1		27,320	(15,101)
Balance, Shares at Mar. 31, 2018		9,051,722	691,209
Balance at Dec. 31, 2017	(1,292)			11,897	(13,189)
Balance, Shares at Dec. 31, 2017		4,980,168	670,090
Net loss	(4,922)
Balance at Jun. 30, 2018	9,287	$ 1		27,397	(18,111)
Balance, Shares at Jun. 30, 2018		9,051,722	691,209
Balance at Mar. 31, 2018	12,220	$ 1		27,320	(15,101)
Balance, Shares at Mar. 31, 2018		9,051,722	691,209
Net loss	(3,010)				(3,010)
Stock-based compensation	77			77
Balance at Jun. 30, 2018	9,287	$ 1		27,397	(18,111)
Balance, Shares at Jun. 30, 2018		9,051,722	691,209
Balance at Dec. 31, 2018	105,068	$ 2		130,776	(25,710)
Balance, Shares at Dec. 31, 2018		22,677,585	727,413
Net loss	(5,742)				(5,742)
Stock-based compensation	181			181
Issuance of common stock upon exercise of stock options	74			74
Issuance of common stock upon exercise of stock options, Shares			164,810
Balance at Mar. 31, 2019	99,581	$ 2		131,031	(31,452)
Balance, Shares at Mar. 31, 2019		22,677,585	892,223
Balance at Dec. 31, 2018	105,068	$ 2		130,776	(25,710)
Balance, Shares at Dec. 31, 2018		22,677,585	727,413
Net loss	$ (13,561)
Issuance of common stock upon exercise of stock options, Shares	194,788
Balance at Jun. 30, 2019	$ 241,536		$ 3	280,804	(39,271)
Balance, Shares at Jun. 30, 2019			32,674,565
Balance at Mar. 31, 2019	99,581	$ 2		131,031	(31,452)
Balance, Shares at Mar. 31, 2019		22,677,585	892,223
Net loss	(7,819)				(7,819)
Stock-based compensation	298			298
Issuance of common stock upon exercise of stock options	27			27
Issuance of common stock upon exercise of stock options, Shares			29,981
Issuance of common stock upon initial public offering, net of underwriting discounts, commissions, and offering costs	149,449		$ 1	149,448
Issuance of common stock upon initial public offering, net of underwriting discounts, commissions, and offering costs, Shares			9,074,776
Conversion of preferred stock to common stock		$ (2)	$ 2
Conversion of preferred stock to common stock, Shares		(22,677,585)	22,677,585
Balance at Jun. 30, 2019	$ 241,536		$ 3	$ 280,804	$ (39,271)
Balance, Shares at Jun. 30, 2019			32,674,565

Condensed consolidated statements of cash flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (13,561)	$ (4,922)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	185	75
Stock-based compensation	479	81
Gain/Loss on disposal of property and equipment	3
Changes in assets and liabilities:
Prepaid expenses and other current assets	(1,021)	(42)
Accounts payable and accrued liabilities	1,095	402
Deferred rent	14	(2)
Net cash used in operating activities	(12,806)	(4,408)
Cash flows from investing activities:
Purchases of property and equipment	(484)	(326)
Proceeds from sale of property and equipment	2
Net cash used in investing activities	(482)	(326)
Cash flows from financing activities:
Proceeds from issuance of convertible Preferred Stock		12,475
Proceeds from issuance of common stock upon exercise of stock options	102	8
Proceeds from the issuance of common stock upon initial public offering	151,912
Payments of initial public offering costs	(1,443)
Other	(1)	(1)
Net cash provided by financing activities	150,570	12,482
Net increase in cash, cash equivalents and restricted cash	137,282	7,748
Cash, cash equivalents and restricted cash—beginning of period	105,603	1,854
Cash, cash equivalents and restricted cash—end of period	242,885	9,602
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment included in accrued expenses and accounts payable	236	91
Deferred offering costs not yet paid	$ 1,021
Issuance of convertible Preferred Stock in exchange for simple agreement for future equity		$ 3,000

Nature of the Business and Basis of Presentation

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Nature of the business and basis of presentation

1. Nature of the business and basis of presentation

Organization

Stoke Therapeutics, Inc. (the Company) was founded in June 2014 and was incorporated under the laws of the State of Delaware. The Company is an early-stage biopharmaceutical company pioneering a new way to treat the underlying causes of severe genetic diseases by precisely upregulating protein expression.

Initial public offering

On June 21, 2019, the Company completed an initial public offering (“IPO”) of its common stock and issued and sold 9,074,776 shares of common stock at a public offering price of $18.00 per share, which included 1,183,666 shares sold upon full exercise of the underwriters’ option to purchase additional shares, resulting in net proceeds of approximately $149.4 million after deducting underwriting discounts, commissions and offering costs of $13.9 million. Upon closing of the IPO, the Company’s outstanding convertible preferred stock automatically converted into shares of common stock (see Note 7). Upon conversion of the convertible preferred stock, the Company reclassified the carrying value of the convertible preferred stock to common stock and additional paid-in capital.

On June 6, 2019, in connection with the IPO, the Company effected a one-for-9.95 reverse split of the Company’s issued and outstanding common and convertible preferred stock. Upon the effectiveness of the reverse stock split, (i) all shares of outstanding common stock and convertible preferred stock were adjusted; (ii) the number of shares of common stock for which each outstanding option to purchase common stock is exercisable were adjusted; and (iii) the exercise price of each outstanding option to purchase common stock were adjusted. All of the outstanding common stock share numbers (including shares of common stock subject to the Company’s options and as converted for the outstanding convertible preferred stock shares), share prices, exercise prices and per share amounts contained in the interim condensed consolidated financial statements have been retroactively adjusted in the interim condensed consolidated financial statements to reflect this reverse stock split for all periods presented. The par value per share and the authorized number of shares of common stock and convertible preferred stock were not adjusted as a result of the reverse stock split.

On June 21, 2019, the Company filed an amended and restated certificate of incorporation with the Secretary of State of the State of Delaware to authorize the issuance of up to 300 million shares of common stock, $0.0001 par value per share, and 10 million shares of undesignated preferred stock, $0.0001 par value per share.

Uncertainties

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 and ability to secure additional capital to fund operations. 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 product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

Liquidity

The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. As of the issuance date of these unaudited condensed consolidated financial statements as of and for the six months ended June 30, 2019, the Company expects that its cash and cash equivalents will be sufficient to fund its operating expenses and capital expenditure requirements into 2023. The future viability of the Company beyond that date is dependent on its ability to raise additional capital to finance its operations.

Summary of Significant Accounting Policies and Recent Accounting Pronouncements

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Summary of significant accounting policies and recent accounting pronouncements

2. Summary of significant accounting policies and recent accounting pronouncements

Basis of presentation and consolidation

The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”), and include the accounts of the Company and its wholly-owned subsidiary. Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). All intercompany transactions between and among its consolidated subsidiary have been eliminated.

Unaudited interim financial information

The accompanying interim unaudited condensed consolidated financial statements and related disclosures are unaudited and have been prepared in accordance with GAAP for interim financial information and the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all the information and footnotes required by GAAP for complete financial statements and should be read in conjunction with the Company’s consolidated financial statements and related footnotes as of and for the year ended December 31, 2018, included in the Company’s prospectus, which was filed with the Securities and Exchange Commission on June 19, 2019. The Company’s financial information as of June 30, 2019, and for the three and six months ended June 30, 2019 and 2018 is unaudited, but in the opinion of management, all adjustments, consisting only of normal recurring adjustments, considered necessary for a fair presentation of the financial position, results of operations and cash flows at the dates and for the periods presented of the results of these interim periods have been included. The balance sheet data as of December 31, 2018 was derived from audited financial statements. The results of the Company’s operations for any interim period are not necessarily indicative of the results that may be expected for any other interim period or for a full fiscal year.

Use of estimates

The preparation of unaudited condensed consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, equity, expenses and disclosure of contingent assets and liabilities. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from Company estimates.

Cash and cash equivalents

The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents. The Company deposits its cash in checking, sweep and money market accounts.

Restricted cash

At June 30, 2019, restricted cash consisted of money market accounts collateralizing letters of credit issued as a security deposits in connection with the Company’s leases of its corporate facilities.

The following table reconciles cash and cash equivalents and restricted cash per the condensed consolidated balance sheet to the condensed consolidated statement of cash flows (in thousands):

	As of June 30,
	2019		2018
Cash and cash equivalents	$	242,680	$	9,546
Restricted cash	205			56
Total cash, cash equivalents and restricted cash	$	242,885	$	9,602

Fair value of financial instruments

Accounting Standards Codification (ASC) Topic 820, Fair Value Measurement (ASC 820), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are those that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances. ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier value hierarchy that distinguishes between the following:

Level 1—Quoted market prices in active markets for identical assets or liabilities.

Level 2—Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates and yield curves.

Level 3—Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use.

To the extent the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair values requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized as Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

Deferred offering costs

The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in the condensed consolidated Statement of stockholders’ equity as a reduction of additional paid-in capital.

Net loss per share

The Company calculates basic and diluted net loss per share attributable to common stockholders in conformity with the two-class method required for participating securities. The Company considers its convertible preferred stock (Preferred Stock) to be participating securities as in the event a dividend is paid on common stock, the holders of Preferred Stock would be entitled to receive dividends on a basis consistent with the common stockholders. Under the two-class method, the net loss attributable to common stockholders is not allocated to the Preferred Stock as the holders of the Preferred Stock do not have a contractual obligation to share in losses.

Under the two-class method, basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock.

Emerging growth company and smaller reporting company status

The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the JOBS Act). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act, until such time as those standards apply to private companies.

The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it is (i) no longer an emerging growth company or (ii) affirmatively and irrevocably opt out of the extended transition period provided in the JOBS Act. As a result, the Company’s unaudited condensed consolidated financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates.

The Company will remain an emerging growth company until the earliest of (1) the last day of its first fiscal year (a) in which the Company has total annual gross revenues of at least $1.07 billion, or (b) in which the Company is deemed to be a large accelerated filer, which means the market value of its common stock that is held by non-affiliates exceeds $700.0 million as of the prior June 30^th, (2) the date on which it has issued more than $1.0 billion in non-convertible debt securities during the prior three-year period and (3) December 31, 2024.

The Company is also a “smaller reporting company,” meaning that as of the date of its initial public offering, the market value of its stock held by non-affiliates is less than $100 million during the most recently completed fiscal year. The Company may continue to be a smaller reporting company as long as either (i) the market value of its stock held by non-affiliates is less than $250 million or (ii) its annual revenue is less than $100 million during the most recently completed fiscal year and the market value of its stock held by non-affiliates is less than $700 million. If the Company is a smaller reporting company at the time it ceases to be an emerging growth company, the Company may continue to rely on exemptions from certain disclosure requirements that are available to smaller reporting companies. Specifically, as a smaller reporting company, the Company may choose to present only the two most recent fiscal years of audited financial statements in its Annual Report on Form 10-K and, similar to emerging growth companies, smaller reporting companies have reduced disclosure obligations regarding executive compensation.

Recently adopted accounting pronouncements

In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting, which provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. The amendments in this ASU should be applied prospectively to an award modified on or after the adoption date. The Company adopted ASU 2017-09 effective January 1, 2018, and the adoption of ASU 2017-09 did not impact the Company’s consolidated financial statements or financial statement disclosures.

Recently issued accounting pronouncements

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), with guidance regarding the accounting for and disclosure of leases. The update requires lessees to recognize all leases, including operating leases, with a term greater than 12 months on the consolidated balance sheet. This update also requires lessees and lessors to disclose key information about their leasing transactions. This guidance was effective for public companies for annual and interim periods beginning after December 15, 2018. For all other entities, this standard is effective for annual reporting periods beginning after December 15, 2019, and interim periods within annual periods beginning after December 15, 2020. The Company intends to adopt this standard on January 1, 2020. While the Company expects the implementation of ASU 2016-02 to result in the recognition of right-of-use assets and lease liabilities for leased facilities, the Company is still evaluating the impact that the adoption of ASU 2016-2 will have on its consolidated financial statements.

In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480) and 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. Part I of this update addresses the complexity of accounting for certain financial instruments with down round features. Down round features are features of certain equity-linked instruments (or embedded features) that result in the strike price being reduced on the basis of the pricing of future equity offerings. Current accounting guidance creates cost and complexity for entities that issue financial instruments (such as warrants and convertible instruments) with down round features that require fair value measurement of the entire instrument or conversion option. Part II of this update addresses the difficulty of navigating Topic 480, Distinguishing Liabilities from Equity, because of the existence of extensive pending content in the FASB Accounting Standards Codification. This pending content is the result of the indefinite deferral of accounting requirements about mandatorily redeemable financial instruments of certain nonpublic entities and certain mandatorily redeemable noncontrolling interests. The amendments in Part II of this update do not have an accounting effect. For public business entities, the amendments in Part I of ASU-2017-11 were effective for fiscal years and interim periods within those years beginning after December 15, 2018. For all other entities, the amendments in Part I of this update are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. The Company intends to adopt Part I on January 1, 2020. Early adoption is permitted for all entities, including adoption in an interim period. The Company is currently assessing the potential impact of adopting ASU 2017-11 on its consolidated financial statements and financial statement disclosures.

In August 2018, the FASB issued ASU 2018-13, “Fair Value Measurement (Topic 820), Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement”. This ASU removed the following disclosure requirements: (1) the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; (2) the policy for timing of transfers between levels; and (3) the valuation processes for Level 3 fair value measurements. Additionally, this update added the following disclosure requirements: (1) the changes in unrealized gains and losses for the period included in other comprehensive income and loss for recurring Level 3 fair value measurements held at the end of the reporting period; (2) the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. For certain unobservable inputs, an entity may disclose other quantitative information (such as the median or arithmetic average) in lieu of the weighted average if the entity determines that other quantitative information would be a more reasonable and rational method to reflect the distribution of unobservable inputs used to develop Level 3 fair value measurements. ASU 2018-13 will be effective for all entities, for fiscal years beginning after December 15, 2019 with early adoption permitted. The Company intends to adopt this standard on January 1, 2020 and does not expect that the adoption of this update will have a material impact on its consolidated financial statements.

Fair Value Measurements

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Fair value measurements

3. Fair value measurements

The following tables present information about the Company’s financial assets 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 as of June 30, 2019
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	242,680	$	—	$	—	$	242,680
Total	$	242,680	$	—	$	—	$	242,680

	Fair value measurements as of December 31, 2018
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	105,399	$	—	$	—	$	105,399
Total	$	105,399	$	—	$	—	$	105,399

The Company’s assets with fair value categorized as Level 1 within the fair value hierarchy include money market funds. Money market funds are publicly traded mutual funds and are presented as cash equivalents on the consolidated balance sheets as of June 30, 2019 and December 31, 2018.

There were no transfers among Level 1, Level 2, or Level 3 categories in the periods presented.

The carrying value of cash, cash equivalents, accounts payable and accrued expenses that are reported on the consolidated balance sheets approximate their fair value due to the short-term nature of these assets and liabilities.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Commitments and Contingencies

6 Months Ended

Jun. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and contingencies

6. Commitments and contingencies

Operating lease

In August 2018, the Company entered into an agreement to sublease approximately 23,000 square feet of space for a term of three years. Lease terms are triple net lease commencing at $0.9 million per year, then with three percent annual base rent increases plus operating expenses, real estate taxes, utilities and janitorial fees. The lease commencement date was December 10, 2018.

In December 2018, the Company entered into an agreement to lease 2,485 square feet of space for a term of three years. The lease includes one renewal option for an additional two years. Lease terms commence at $0.2 million per annum, with 2.5% annual base rent increases plus operating expenses, real estate taxes, utilities and janitorial fees. The Company first occupied this space in May 2019.

As of June 30, 2019, the future minimum payments for operating leases are as follows (in thousands):

July 1, 2019 to December 31, 2019	$	561
2020		1,149
2021		1,102
2022		81
Thereafter		—
	$	2,893

Rent expense incurred under operating leases was approximately $0.26 million and $0.07 million for the three months ended June 30, 2019 and 2018, respectively and $0.5 million and $0.1 million for the six months ended June 30, 2019 and 2018 respectively.

Consulting agreement

In October 2014, the Company entered into a consulting agreement with a member of the Company’s board of directors, who is also an employee of Cold Spring Harbor Laboratory (“CSHL”), to provide consulting services related to scientific research related to the development of antisense-based drugs, therapies, diagnostic and research tools, products, services and intellectual property. The Company recognized expense of $0.02 million and $0.02 million in the three month period ended June 30, 2019 and 2018 respectively, and $0.05 million and $0.05 million in the six month period ended June 30, 2019 and 2018 respectively, for such consulting services. The initial term of this agreement was five years and may be extended by the mutual consent of the Company and the board member.

License and research agreements

In July 2015, the Company entered into a worldwide license agreement (the “CSHL Agreement”), with CSHL, with respect to Targeted Augmentation of Nuclear Gene Output (“TANGO”) patents. Under the CSHL Agreement, the Company receives an exclusive (except with respect to certain government rights and non-exclusive licenses), worldwide license under certain patents and applications relating to TANGO. As part of the CSHL Agreement, the Company granted CSHL 164,927 shares of common stock valued based on an independent appraisal at approximately $0.07 million. The CSHL Agreement obligates the Company to make additional payments that are contingent upon certain milestones being achieved. The Company is also required to pay royalties, tiered based on the scope of patent coverage for each licensed product, ranging from a low-single digit percentage to a mid-single digit percentage on annual net sales. These royalty obligations apply on a licensed product-by-licensed product and country-by-country basis until the latest of (i) the expiration of the last valid claim of a CSHL patent covering the applicable licensed product or (ii) the expiration of any regulatory exclusivity for the applicable licensed product. In addition, if the Company sublicenses the rights under the CSHL Agreement, the Company is required to pay a maximum of twenty percent of the sublicense revenue to CSHL, which may be reduced to a mid-teens or a mid-single digit percentage upon achievement of certain clinical milestones for the applicable licensed product. Finally, the Company is required to pay an annual license maintenance fee of $0.01 million, which amount is creditable against any owed royalty or milestone payments. The maximum aggregate potential milestone payments payable total approximately $0.9 million. Additionally, certain licenses under the CSHL Agreement require the Company to reimburse CSHL for certain past and ongoing patent related expenses, however there were no expenses related to these reimbursable patent costs during the three months and six months ended June 30, 2019 and 2018.

In April 2016, the Company entered into an exclusive, worldwide license agreement with the University of Southampton, (the “Southampton Agreement”), whereby the Company acquired rights to foundational technologies related to the Company’s TANGO technology. Under the Southampton Agreement, the Company receives an exclusive, worldwide license under certain licensed patents and applications relating to TANGO. As part of the Southampton Agreement, the Company paid 0.06 million pounds sterling (approximately $0.07 million as of the date thereof) as an up-front license fee. Under the Southampton Agreement, the Company may be obligated to make additional payments that are contingent upon certain milestones being achieved, as well as royalties on future product sales. These royalty obligations survive until the latest of (i) the expiration of the last valid claim of a licensed patent covering a subject product or (ii) the expiration of any regulatory exclusivity for the subject product in a country. In addition, if the Company sublicenses its rights under the Southampton Agreement, the Company is required to pay a mid-single digit percentage of the sublicense revenue to the University of Southampton. The maximum aggregate potential milestone payments payable by the Company totaled approximately 0.4 million pounds sterling (approximately $0.5 million as of June 30, 2019). As of June 30, 2019, and December 31, 2018, the Company had recorded no liabilities under the Southampton Agreement.

Sponsored research agreement

In December 2017, the Company entered into a sponsored research agreement with the University of Michigan (the “University of Michigan Agreement”) to fund research conducted at the University of Michigan through November 2018. The budget for the scope of work described in the research agreement was $0.4 million. In December 2018, the University of Michigan Agreement was renewed and extended through November 2020 with a budget for the scope of work of $0.6 million. The costs incurred by the Company under the University of Michigan Agreement are recorded as research and development expense and expensed in the period in which they are incurred. Research and development spending under the University of Michigan Agreement was $0.16 million and $0.09 million for the three months ended June 30, 2019 and 2018, respectively, and $0.2 million and $0.1 million for the six months ended June 30, 2019 and 2018, respectively.

Convertible Preferred Stock

6 Months Ended

Jun. 30, 2019

Stockholders Equity Note [Abstract]

Convertible preferred stock

7. Convertible preferred stock

On June 6, 2019, the Company’s stockholders approved an amendment to the Company’s Amended and Restated Certificate of Incorporation which became effective on June 21, 2019 upon which the authorized capital stock of the Corporation consisted of 300 million shares of common stock, $0.0001 per share par value, and 10 million shares of undesignated preferred stock, $0.0001 per share par value.

On June 21, 2019, the Company completed its IPO and in conjunction therewith, all outstanding convertible preferred stock was converted into common stock. Accordingly, as of June 30, 2019, there was no preferred stock outstanding. At December 31, 2018, the Company’s amended and restated certificate of incorporation authorized the Company to issue 225,584,874 shares of $0.0001 par value convertible preferred stock, 49,540,132 were designated Series A convertible preferred stock (Series A Preferred), 75,777,370 were designated Series A-2 Preferred and 100,267,372 were designated Series B convertible preferred stock (Series B Preferred) (collectively, the Preferred Stock). The following table summarizes the Company’s issued and outstanding Preferred Stock (in thousands except share amounts):

	Series A Preferred				Series A-2 Preferred				Series B Preferred				Total convertible Preferred
	Shares		Amount		Shares		Amount		Shares		Amount		Shares		Amount
Balance, December 31, 2018		4,980,168	$	11,725		7,617,746	$	28,880		10,079,671	$	89,742		22,677,585	$	130,347

Equity Incentive Plans

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity incentive plans

8. Equity incentive plans

In June 2019, the Company’s board of directors and stockholders approved the 2019 Equity Incentive Plan (the “2019 Plan”) which became effective on June 17, 2019 and replaced the Company’s 2014 Equity Incentive Plan (the “2014 Plan”). In addition to the shares of common stock reserved for future issuance under the 2014 Plan that were added to the 2019 Plan upon its effective date, the Company initially reserved 2,200,000 shares of common stock for issuance under the 2019 Plan. The number of shares reserved for issuance under the Company’s 2019 Plan will increase automatically on January 1 of each of 2020 through 2029 by the number of shares equal to 4% of the aggregate number of outstanding shares of the Company’s common stock as of the immediately preceding December 31, or a lesser number as may be determined by the Company’s board of directors.

As of June 30, 2019, there were no shares available for future issuance under the 2014 Plan and 2,413,594 shares were available under the 2019 Plan.

During the six months ended June 30, 2019, the Company granted options to purchase 906,203 shares of common stock to certain of its employees, and directors, respectively. The options vest over four years and are exercisable at a per share price equal to the fair value of the common stock on the grant date.

The Company determined the volatility for options granted in 2019 based on a study of reported data for a guideline group of companies that issued options with substantially similar terms. The risk-free interest rate is based on a zero-coupon United States Treasury instrument with terms consistent with the expected life of the stock options. The Company has not paid and does not anticipate paying cash dividends on shares of common stock; therefore, the expected dividend yield is assumed to be zero.

A summary of stock option activity for awards is presented below:

	Number of shares			Weighted average exercise price		Weighted average remaining contractual life (years)		Aggregate intrinsic value⁽¹⁾
Outstanding as of December 31, 2018		3,484,628		$	1.20		8.9	$	3,436
Granted		906,203			4.67		—		—
Exercised		(194,788	)		0.53		—		—
Forfeited or expired		(9,652)			0.55		—		—
Outstanding as of June 30, 2019		4,186,391		$	1.99		8.7	$	113,796
Exercisable as of June 30, 2019		1,672,557		$	0.71		7.9	$	47,600

(1)	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at June 30, 2019 (in thousands).

The weighted average grant date fair value per share of stock options granted during the six months ended June 30, 2019 was $4.67. The aggregate intrinsic value of stock options exercised during the six months ended June 30, 2019 was $5.6 million.

The aggregate grant date fair value of stock options granted during the six months ended June 30, 2019 was approximately $4.2 million.

Stock-based compensation

As of June 30, 2019, there was $4.0 million of unrecognized compensation cost related to unvested stock-based compensation arrangements granted under the 2014 Plan. The compensation is expected to be recognized over a weighted average period of 4 years as of June 30, 2019.

Stock-based compensation expense recorded as research and development and general and administrative expenses in the accompanying condensed consolidated statements of operations is as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	121	$	25	$	195	$	26
General and administrative		177		52		284		55
	$	298	$	77	$	479	$	81

The Company uses the Black-Scholes option pricing model to calculate the grant-date fair value of an award. The fair values of the options granted to employees and directors were calculated using the following assumptions for the three months and six months ended June 30, 2019 and 2018:

	Three Months Ended June 30,		Six Months Ended June 30,
	2019	2018	2019	2018
Risk-free interest rate	2.01% - 2.42%	2.67%	2.01% - 2.81%	2.67%
Expected dividend yield	—	—	—	—
Expected life	6.375 years	6.25 - 6.375 years	6.375 years	6.25 - 6.375 years
Expected volatility	61% - 68%	57%	60% - 68%	57%

2019 Employee stock purchase plan

In June 2019, the Company adopted the 2019 Employee Stock Purchase Plan (“ESPP”), which became effective on June 18, 2019. The Company initially reserved 315,000 shares of common stock for sale under the ESPP. The number of shares reserved for issuance under the ESPP will increase automatically on January 1st of each of the first ten calendar years following the first offering date by the number of shares equal to the lesser of 1% of the total outstanding shares of the Company’s common stock as of the immediately preceding December 31 or a lower amount determined by the Company’s board of directors. The aggregate number of shares issued over the term of the ESPP will not exceed 3,150,000 shares of the Company’s common stock.

Net Loss Per Share Attributable to Common Stockholders

6 Months Ended

Jun. 30, 2019

Earnings Per Share [Abstract]

Net loss per share attributable to common stockholders

9. Net loss per share attributable to common stockholders

The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands except share and per share amounts):

	Three Months Ended June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Numerator:
Net loss	$	(7,819	)	$	(3,010	)	$	(13,561	)	$	(4,922	)
Denominator:
Weighted-average number of common shares, basic and diluted		5,083,620			691,212			2,970,292			689,112
Net loss per common share attributable to common stockholders, basic and diluted	$	(1.54	)	$	(4.36	)		(4.57	)		(7.14	)

The Company’s potential dilutive securities, which include Preferred Stock and common stock options, have been excluded from the computation of diluted net loss per share as the effect would be anti-dilutive. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at period end, from the computation of diluted net loss per share attributable to common stockholders for the period indicated because including them would have had an anti-dilutive effect:

	June 30,
	2019		2018
Preferred Stock		—		9,051,722
Outstanding options to purchase common stock		4,186,391		2,081,131
		4,186,391		11,132,853

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Policies)

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Basis of Presentation and Consolidation

Basis of presentation and consolidation

Unaudited Interim Financial Information

Unaudited interim financial information

Use of Estimates

Use of estimates

Cash and Cash Equivalents

Cash and cash equivalents

Restricted Cash

Restricted cash

The following table reconciles cash and cash equivalents and restricted cash per the condensed consolidated balance sheet to the condensed consolidated statement of cash flows (in thousands):

	As of June 30,
	2019		2018
Cash and cash equivalents	$	242,680	$	9,546
Restricted cash	205			56
Total cash, cash equivalents and restricted cash	$	242,885	$	9,602

Fair Value of Financial Instruments

Fair value of financial instruments

Level 1—Quoted market prices in active markets for identical assets or liabilities.

Level 2—Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates and yield curves.

Level 3—Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use.

Deferred Offering Costs

Deferred offering costs

Net Loss per Share

Net loss per share

Emerging Growth Company and Smaller Reporting Company Status

Emerging growth company and smaller reporting company status

Recently adopted accounting pronouncements

Recently Adopted Accounting Pronouncements

Recently adopted accounting pronouncements

Recently Issued Accounting Pronouncements

Recently issued accounting pronouncements

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Schedule of Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Utilized

	Fair value measurements as of June 30, 2019
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	242,680	$	—	$	—	$	242,680
Total	$	242,680	$	—	$	—	$	242,680

	Fair value measurements as of December 31, 2018
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	105,399	$	—	$	—	$	105,399
Total	$	105,399	$	—	$	—	$	105,399

Equity Incentive Plans (Tables)

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity for Awards

A summary of stock option activity for awards is presented below:

	Number of shares			Weighted average exercise price		Weighted average remaining contractual life (years)		Aggregate intrinsic value⁽¹⁾
Outstanding as of December 31, 2018		3,484,628		$	1.20		8.9	$	3,436
Granted		906,203			4.67		—		—
Exercised		(194,788	)		0.53		—		—
Forfeited or expired		(9,652)			0.55		—		—
Outstanding as of June 30, 2019		4,186,391		$	1.99		8.7	$	113,796
Exercisable as of June 30, 2019		1,672,557		$	0.71		7.9	$	47,600

(1)	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at June 30, 2019 (in thousands).

Summary of Stock-based Compensation Expense

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	121	$	25	$	195	$	26
General and administrative		177		52		284		55
	$	298	$	77	$	479	$	81

Summary of Fair Values of Options Granted to Employees and Directors

	Three Months Ended June 30,		Six Months Ended June 30,
	2019	2018	2019	2018
Risk-free interest rate	2.01% - 2.42%	2.67%	2.01% - 2.81%	2.67%
Expected dividend yield	—	—	—	—
Expected life	6.375 years	6.25 - 6.375 years	6.375 years	6.25 - 6.375 years
Expected volatility	61% - 68%	57%	60% - 68%	57%

Net Loss Per Share Attributable to Common Stockholders (Tables)

6 Months Ended

Jun. 30, 2019

Earnings Per Share [Abstract]

Schedule of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders

The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands except share and per share amounts):

	Three Months Ended June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Numerator:
Net loss	$	(7,819	)	$	(3,010	)	$	(13,561	)	$	(4,922	)
Denominator:
Weighted-average number of common shares, basic and diluted		5,083,620			691,212			2,970,292			689,112
Net loss per common share attributable to common stockholders, basic and diluted	$	(1.54	)	$	(4.36	)		(4.57	)		(7.14	)

Schedule of Antidilutive Securities Excluded from Computation of Diluted Net Loss Per Share

The Company excluded the following potential common shares, presented based on amounts outstanding at period end, from the computation of diluted net loss per share attributable to common stockholders for the period indicated because including them would have had an anti-dilutive effect:

	June 30,
	2019		2018
Preferred Stock		—		9,051,722
Outstanding options to purchase common stock		4,186,391		2,081,131
		4,186,391		11,132,853

Nature of the Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Millions			3 Months Ended	6 Months Ended
	Jun. 21, 2019 USD ($) $ / shares shares	Jun. 06, 2019 $ / shares shares	Jun. 30, 2019 $ / shares shares	Jun. 30, 2019 $ / shares shares	Dec. 31, 2018 $ / shares shares
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporation state name				Delaware
Reverse stock split, description				On June 6, 2019, in connection with the IPO, the Company effected a one-for-9.95 reverse split of the Company’s issued and outstanding common and convertible preferred stock.
Reverse stock split		0.1005
Common stock, shares authorized	300,000,000	300,000,000	300,000,000	300,000,000	300,000,000
Common stock, per share par value \| $ / shares	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Undesignated preferred stock, shares authorized	10,000,000	10,000,000	10,000,000	10,000,000	0
Undesignated preferred stock, per share par value \| $ / shares	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Common Stock
Nature Of Business And Basis Of Presentation [Line Items]
Stock issued during period, shares			9,074,776
Net proceeds from initial public offering \| $	$ 149.4
Initial Public Offering \| Common Stock
Nature Of Business And Basis Of Presentation [Line Items]
Stock issued during period, shares	9,074,776
Stock issued, price per share \| $ / shares	$ 18.00
Offering costs \| $	$ 13.9
Underwriters’ Option \| Common Stock
Nature Of Business And Basis Of Presentation [Line Items]
Stock issued during period, shares	1,183,666

Commitments and Contingencies - Additional Information (Details)	1 Months Ended					3 Months Ended		6 Months Ended
	Dec. 31, 2018 USD ($) ft² Option	Aug. 31, 2018 USD ($) ft²	Jul. 31, 2015 USD ($) shares	Jun. 30, 2014 USD ($)	Jun. 30, 2014 GBP (£)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 GBP (£)	Dec. 31, 2017 USD ($)
Commitments And Contingencies [Line Items]
Area subject to sublease \| ft²		23,000
Term of sublease		3 years
Sublease for year		$ 900,000
Annual base rent increase percentage for sublease		3.00%
Operating lease commencement date		Dec. 10, 2018
Area of space subject to operating lease \| ft²	2,485
Operating lease term	3 years
Number of options to renewal operating lease term \| Option	1
Operating lease, renewal term	2 years
Operating lease cost per year	$ 200,000
Annual base rent increase percentage for operating lease	2.50%
Operating leases, rent expense						$ 260,000	$ 70,000	$ 500,000	$ 100,000
Shares of common stock granted, value						149,449,000
Liabilities recorded under agreement	$ 2,471,000					4,836,000		4,836,000
The University of Michigan Agreement
Commitments And Contingencies [Line Items]
Budget for research agreement	600,000										$ 400,000
Research and development spending						160,000	90,000	200,000	100,000
CSHL Agreement
Commitments And Contingencies [Line Items]
Stock issued during period, shares \| shares			164,927
Shares of common stock granted, value			$ 70,000
Percentage of maximum required payment of sublicense revenue			20.00%
Annual license maintenance fee								10,000
Maximum aggregate potential milestone payments payable						900,000		900,000
Expenses related to reimbursable patent costs						0	0	0	0
Southampton Agreement
Commitments And Contingencies [Line Items]
Maximum aggregate potential milestone payments payable						500,000		500,000		£ 400,000
Payment of up-front license fee				$ 70,000	£ 60,000
Liabilities recorded under agreement	$ 0					0		0
Member of Board of Directors
Commitments And Contingencies [Line Items]
Consulting services expenses						$ 20,000	$ 20,000	$ 50,000	$ 50,000
Consulting services agreement term								5 years

Equity Incentive Plans - Additional Information (Details) $ / shares in Units, $ in Millions	1 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($) shares	Jun. 30, 2019 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted to purchase shares		906,203
Options vest over period		4 years
Expected dividend yield		0.00%
Weighted average grant date fair value per share of stock options granted \| $ / shares		$ 4.67
Aggregate intrinsic value of stock options exercised \| $		$ 5.6
Aggregate grant date fair value of stock options granted \| $		4.2
Unrecognized compensation cost \| $	$ 4.0	$ 4.0
Expected weighted average period		4 years
2019 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares of common stock reserved for issuance	2,200,000	2,200,000
Percentage of increase in reserved common shares	4.00%
Number of common shares available for future issuance	2,413,594	2,413,594
The 2014 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares available for future issuance	0	0
2019 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares available for future issuance	315,000	315,000
2019 ESPP \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of increase in reserved common shares	1.00%
Number of common shares available for future issuance	3,150,000	3,150,000

Equity Incentive Plans - Summary of Stock Option Activity for Awards (Details)
$ / shares in Units, $ in Thousands

6 Months Ended

12 Months Ended

Jun. 30, 2019

USD ($)

$ / shares

Dec. 31, 2018

USD ($)

$ / shares

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Number of shares, Outstanding as of December 31, 2018 | shares

3,484,628

Number of shares, Granted | shares

906,203

Number of shares, Exercised | shares

(194,788)

Number of shares, Forfeited or expired | shares

(9,652)

Number of shares, Outstanding as of June 30, 2019 | shares

4,186,391

3,484,628

Number of shares, Exercisable as of June 30, 2019 | shares

1,672,557

Weighted average exercise price, Outstanding as of December 31, 2018 | $ / shares

$ 1.20

Weighted average exercise price, Granted | $ / shares

4.67

Weighted average exercise price, Exercised | $ / shares

0.53

Weighted average exercise price, Forfeited or expired | $ / shares

0.55

Weighted average exercise price, Outstanding as of June 30, 2019 | $ / shares

1.99

$ 1.20

Weighted average exercise price, Exercisable as of June 30, 2019 | $ / shares

$ 0.71

Weighted average remaining contractual life, Outstanding

8 years 8 months 12 days

8 years 10 months 24 days

Weighted average remaining contractual life, Exercisable

7 years 10 months 24 days

Aggregate intrinsic value, Outstanding value | $

$ 113,796

^[1]

$ 3,436

^[1]

Aggregate intrinsic value, Exercisable as of June 30, 2019 | $

$ 47,600

^[1]

[1]	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at June 30, 2019 (in thousands).

Equity Incentive Plans - Summary of Fair Values of Options Granted to Employees and Directors (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate		2.67%		2.67%
Risk-free interest rate, minimum	2.01%		2.01%
Risk-free interest rate, maximum	2.42%		2.81%
Expected dividend yield			0.00%
Expected life	6 years 4 months 15 days		6 years 4 months 15 days
Expected volatility		57.00%		57.00%
Expected volatility, minimum	61.00%		60.00%
Expected volatility, maximum	68.00%		68.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life		6 years 3 months		6 years 3 months
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life		6 years 4 months 15 days		6 years 4 months 15 days

Condensed consolidated statements of stockholders' equity (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Statement Of Stockholders Equity [Abstract]
Stock issuance cost	$ 88

Accrued and Other Current Liabilities - Schedule of Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued employee compensation costs	$ 882	$ 901
Accrued professional	988	200
Accrued research and development costs	1,677	234
Accrued other	33	61
Total accrued and other current liabilities	$ 3,580	$ 1,396

Simple Agreement For Future Equity - Additional Information (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jan. 31, 2018	Oct. 31, 2017
Class Of Stock [Line Items]
Preferred stock, shares issued	0
Simple Agreement For Future Equity
Class Of Stock [Line Items]
Future equity financing				$ 3.0
Series A-2 Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares issued		7,617,746
Series A-2 Preferred Stock \| Simple Agreement For Future Equity
Class Of Stock [Line Items]
Preferred stock, shares issued			788,042

Convertible Preferred Stock - Summary of Issued and Outstanding Preferred Stock (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Shares issued	0
Shares outstanding	0
Series A Convertible Preferred Stock
Class Of Stock [Line Items]
Shares issued		4,980,168
Shares outstanding		4,980,168
Preferred stock amount		$ 11,725
Series A-2 Convertible Preferred Stock
Class Of Stock [Line Items]
Shares issued		7,617,746
Shares outstanding		7,617,746
Preferred stock amount		$ 28,880
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Shares issued		10,079,671
Shares outstanding		10,079,671
Preferred stock amount		$ 89,742
Convertible Preferred Stock
Class Of Stock [Line Items]
Shares issued	0	22,677,585
Shares outstanding	0	22,677,585
Preferred stock amount		$ 130,347

Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Utilized (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	$ 242,680	$ 105,399
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	242,680	105,399
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	242,680	105,399
Money Market Funds \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	$ 242,680	$ 105,399

Fair Value Measurements - Additional Information (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value assets, transfers between level 1, level 2 and level 3, amount	$ 0	$ 0
Fair value liabilities, transfers between level 1, level 2 and level 3, amount	$ 0	$ 0

Commitments and Contingencies - Schedule of Future Minimum Payments for Operating Leases (Details) $ in Thousands	Jun. 30, 2019 USD ($)
Future minimum payments for operating leases
July 1, 2019 to December 31, 2019	$ 561
2020	1,149
2021	1,102
2022	81
Total	$ 2,893

Net Loss Per Share Attributable to Common Stockholders - Schedule of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (7,819)	$ (5,742)	$ (3,010)	$ (1,912)	$ (13,561)	$ (4,922)
Denominator:
Weighted-average common shares outstanding, basic and diluted	5,083,620		691,212		2,970,292	689,112
Net loss per share attributable to common stockholders, basic and diluted	$ (1.54)		$ (4.36)		$ (4.57)	$ (7.14)

Net Loss Per Share Attributable to Common Stockholders - Schedule of Antidilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	4,186,391	11,132,853
Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount		9,051,722
Outstanding Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	4,186,391	2,081,131