STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (15,487)	$ (12,019)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	79	72
Stock-based compensation	2,024	2,009
Change in fair value of derivative liabilities	1,560
Issuance of common stock in a license agreement		87
(Increase) decrease in:
Receivables	(46)	(614)
Prepaid expenses and other current assets	(2,426)	(3,125)
Other noncurrent assets		(5)
Increase (decrease) in:
Accounts payable	70	(100)
Accrued expenses	1,577	(324)
Deferred revenue	(1,597)	(1,969)
Deferred rent	(31)	(91)
Net cash used in operating activities	(14,277)	(16,079)
Cash flows from investing activities:
Purchases of property and equipment	(358)	(157)
Net cash used in investing activities	(358)	(157)
Cash flows from financing activities:
Proceeds from exercise of stock options	29	367
Repurchase of restricted common stock		(2)
Net cash provided by financing activities	29	365
Net decrease in cash and cash equivalents	(14,606)	(15,871)
Cash and cash equivalents, beginning of period	81,053	140,717
Cash and cash equivalents, end of period	66,447	124,846
Supplementary disclosure of cash flow information:
Cash paid for interest	0	0
Cash paid for income taxes	0	$ 0
Series 1 Preferred Stock
Supplementary disclosure of noncash investing and financing activities:
Payment of dividends in preferred stock	$ 4,171