CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (48,079)	$ (20,563)	$ (8,517)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	120	90	12
Stock-based compensation expense	6,104	891	27
Write-down of property and equipment held for sale	216	289
Non-cash charges related to Vixen acquisition	2,784
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(8)	(1,269)	(152)
Accounts payable	1,810	(359)	819
Accrued expenses	2,450	552	175
Net cash used in operating activities	(34,603)	(20,369)	(7,636)
Cash flows from investing activities:
Purchases of property and equipment	(232)	(507)	(417)
Purchases of marketable securities	(148,764)	(82,513)	(5,035)
Proceeds from sales and maturities of marketable securities	87,093	6,069	3,673
Net cash used in investing activities	(61,903)	(76,951)	(1,779)
Cash flows from financing activities:
Proceeds from issuance of common stock in connection with private placement, net of issuance costs	18,547
Proceeds from follow-on public offering, net of issuance costs	98,158
Proceeds from initial public offering, net of offering costs		56,550
Proceeds from issuance of convertible preferred stock, net of issuance costs		39,864	10,584
Proceeds from the exercise of employee stock options	121
Net cash provided by financing activities	116,826	96,414	10,584
Net increase (decrease) in cash and cash equivalents	20,320	(906)	1,169
Cash and cash equivalents at beginning of period	9,851	10,757	9,588
Cash and cash equivalents at end of period	30,171	9,851	10,757
Supplemental disclosures of non-cash investing and financing activities:
Additions to property and equipment included in accounts payable	11	2	91
Fair value of stock issued in connection with Vixen acquisition	2,355
Offering costs included in accounts payable	$ 250
Accretion of convertible preferred stock to redemption value		$ 1,764	2,054
Fair value of preferred stock purchased put option on date of issuance			$ 1,039