CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (12,559)	$ (13,039)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	50	21
Stock-based compensation expense	3,153	1,222
Non-cash charges related to Vixen acquisition		2,784
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2,597)	71
Accounts payable	831	2,008
Accrued expenses	(1,537)	39
Net cash used in operating activities	(12,659)	(6,894)
Cash flows from investing activities:
Purchases of property and equipment	(195)	(40)
Purchases of marketable securities	(17,158)
Proceeds from sales and maturities of marketable securities	23,309	10,214
Net cash provided by investing activities	5,956	10,174
Cash flows from financing activities:
Proceeds from the exercise of employee stock options	209
Net cash provided by financing activities	209
Net (decrease) increase in cash and cash equivalents	(6,494)	3,280
Cash and cash equivalents at beginning of period	30,171	9,851
Cash and cash equivalents at end of period	23,677	13,131
Supplemental disclosures of non-cash investing and financing activities:
Additions to property and equipment included in accounts payable	$ 91	40
Fair value of stock issued in connection with Vixen acquisition		$ 2,355