Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (15,739)	$ (13,681)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	85	190
Net amortization of premiums and discounts on investments	(88)	(14)
Stock-based compensation expense	1,084	519
Change in deferred rent	(11)	(29)
Loss on disposal of property and equipment		12
Write-off of deferred offering costs		1,500
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(909)	(336)
Other assets	33
Accounts payable	(233)	284
Accrued expenses and other current liabilities	1,085	774
Net cash used in operating activities	(14,693)	(10,781)
Cash flows from investing activities:
Purchases of property and equipment	(74)
Purchases of investments	(52,130)	(12,634)
Proceeds from sales or maturities of investments	8,775	11,150
Changes in restricted cash		(25)
Net cash used in investing activities	(43,429)	(1,509)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	471	13,183
Proceeds from exercise of stock options	100	23
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	52,313
Payments of initial public offering costs	(2,049)	(180)
Net cash provided by financing activities	50,835	13,026
Net increase (decrease) in cash and cash equivalents	(7,287)	736
Cash and cash equivalents at beginning of period	20,715	3,768
Cash and cash equivalents at end of period	13,428	4,504
Supplemental disclosure of non-cash financing activities:
Accretion of redeemable convertible preferred stock to redemption value	41	$ 57
Deferred offering costs included in accounts payable and accrued expenses	255
Conversion of convertible preferred stock into common stock upon listing of the Company's common stock on the NASDAQ	$ 130,412