CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (27,397)	$ (25,910)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	105	48
Stock-based compensation expense	6,457	2,576
Non-cash charges related to Vixen acquisition		2,784
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(3,897)	30
Accounts payable	3,161	2,493
Accrued expenses	(1,168)	1,353
Net cash used in operating activities	(22,739)	(16,626)
Cash flows from investing activities:
Purchases of property and equipment	(388)	(106)
Purchases of marketable securities	(41,534)	(11,282)
Proceeds from sales and maturities of marketable securities	47,652	31,430
Net cash provided by investing activities	5,730	20,042
Cash flows from financing activities
Proceeds from issuance of common stock in connection with private placement, net of issuance costs		18,547
Proceeds from the exercise of employee stock options	235	1
Net cash provided by financing activities	19,546	18,548
Net increase in cash and cash equivalents	2,537	21,964
Cash and cash equivalents at beginning of period	30,171	9,851
Cash and cash equivalents at end of period	32,708	31,815
Supplemental disclosures of non-cash investing and financing activities:
Additions to property and equipment included in accounts payable	$ 190	18
Fair value of stock issued in connection with Vixen acquisition		$ 2,355