STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net loss	$ (16,020)	$ (15,487)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	117	79
Stock-based compensation	3,659	2,024
Change in fair value of derivative liabilities	(28)	1,560
(Increase) decrease in:
Receivables	1	(46)
Prepaid expenses and other current assets	(1,172)	(2,426)
Other noncurrent assets	(490)
Increase (decrease) in:
Accounts payable	(3,811)	70
Accrued expenses	(1,030)	1,577
Contract liabilities	(146)	(1,597)
Deferred rent	(37)	(31)
Net cash used in operating activities	(18,957)	(14,277)
Cash flows from investing activities:
Purchases of property and equipment	(150)	(358)
Net cash used in investing activities	(150)	(358)
Cash flows from financing activities:
Proceeds from exercise of stock options		29
Repurchase of restricted common stock	(731)
Net cash provided by (used in) financing activities	(731)	29
Net decrease in cash, cash equivalents, and restricted cash	(19,838)	(14,606)
Cash, cash equivalents, and restricted cash, beginning of period	71,335	81,441
Cash equivalents, and restricted cash, end of period	51,497	66,835
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	$ 5,120	$ 4,171