Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (26,268)	$ (24,983)	$ (15,834)
Adjustment to reconcile net loss to net cash used in operating activities:
Restructuring charges	4,442	0	0
Depreciation and amortization	159	54	35
Stock-based compensation	1,592	985	408
Issuance of stock for services	0	0	4
Amortization of premiums/ discounts on marketable securities	260	0	0
Loss on disposal of assets	128	0	0
Changes in operating assets and liabilities:
Grant reimbursement and other receivables	(160)	119	40
Prepaid expenses and other current assets	107	(650)	(99)
Deferred offering costs	0	0	105
Other noncurrent assets	0	0	17
Accounts payable	(3,264)	2,816	189
Accrued expenses	(1,801)	524	1,165
Net cash used in operating activities	(24,805)	(21,135)	(13,970)
Investing activities
Purchases of marketable securities	(103,114)	0	0
Maturities of marketable securities	58,784	0	0
Restricted cash	(2,432)	0	0
Purchase of property and equipment	(1,729)	(251)	(102)
Net cash used in investing activities	(48,491)	(251)	(102)
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	41,433	0
Proceeds from the issuance of common stock, net of issuance costs	0	60,280	0
Proceeds from the exercise of stock options	16	67	209
Cash provided by financing activities	16	101,780	209
Net increase (decrease) in cash and cash equivalents	(73,280)	80,394	(13,863)
Cash and cash equivalents at beginning of period	89,713	9,319	23,182
Cash and cash equivalents at end of period	16,432	89,713	9,319
Supplemental disclosure of non-cash investing and financing activities
Conversion of preferred stock to common stock	$ 0	$ 100,938	$ 0