CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net loss	$ (20,563)	$ (8,517)	$ (5,236)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	90	12	11
Stock-based compensation expense	891	27
Deferred rent, amortization of deferred credits	(5)
Deferred rent, amortization of deferred charges		1	3
Write-down of property and equipment held for sale	289
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,269)	(152)	7
Accounts payable	(359)	819	303
Accrued expenses	557	174	(8)
Net cash used in operating activities	(20,369)	(7,636)	(4,920)
Cash flows from investing activities:
Purchases of property and equipment	(507)	(417)
Purchases of marketable securities	(82,513)	(5,035)	(4,535)
Proceeds from sales and maturities of marketable securities	6,069	3,673
Net cash used in investing activities	(76,951)	(1,779)	(4,535)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	39,864	10,584
Proceeds from initial public offering, net of offering costs	56,550
Net cash provided by financing activities	96,414	10,584
Net (decrease) increase in cash and cash equivalents	(906)	1,169	(9,455)
Cash and cash equivalents at beginning of period	10,757	9,588	19,043
Cash and cash equivalents at end of period	9,851	10,757	9,588
Supplemental disclosures of non-cash investing and financing activities:
Additions to property and equipment purchases included in accounts payable	2	91
Accretion of convertible preferred stock to redemption value	$ 1,764	2,054	$ 1,740
Fair value of preferred stock purchased put option on date of issuance		$ 1,039