CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities:
Net loss	$ (36,604)	$ (15,849)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	80	57
Stock-based compensation expense	4,194	243
Non-cash charges related to Vixen acquisition	2,784
Changes in operating assets and liabilities:
Prepaid expenses and other assets	110	(764)
Accounts payable	915	(491)
Accrued expenses	1,990	572
Net cash used in operating activities	(26,531)	(16,232)
Cash flows from investing activities:
Purchases of property and equipment	(170)	(375)
Purchases of marketable securities	(33,747)	(13,002)
Proceeds from sales and maturities of marketable securities	49,832	5,914
Net cash provided by (used in) investing activities	15,915	(7,463)
Cash flows from financing activities:
Proceeds from issuance of common stock in connection with private placement, net of issuance costs	18,547
Proceeds from issuance of convertible preferred stock, net of issuance costs		39,864
Proceeds from the exercise of employee stock options	14
Payment of deferred offering costs		(1,507)
Net cash provided by financing activities	18,561	38,357
Net increase in cash and cash equivalents	7,945	14,662
Cash and cash equivalents at beginning of period	9,851	10,757
Cash and cash equivalents at end of period	17,796	25,419
Supplemental disclosures of non-cash investing and financing activities:
Additions to property and equipment included in accounts payable	13	19
Accretion of convertible preferred stock to redemption value		1,764
Fair value of stock issued in connection with Vixen acquisition on date of issuance	$ 2,355
Deferred offering costs included in accounts payable		$ 436