CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash Flows from Operating Activities:
Net loss	$ (15,057)	$ (12,823)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	1,784	1,716
Issuance of common stock for services rendered	43	32
Accretion of premiums and discounts on investments	74	195
Depreciation and amortization expense	176	157
Other		3
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,205)	(46)
Accounts payable, accrued expenses, and other liabilities	(2,183)	(1,812)
Deferred revenue	(278)	(277)
Net cash used in operating activities	(17,646)	(12,855)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities		(2,946)
Proceeds from maturity of available-for-sale securities	16,720	9,239
Proceeds from sale of available-for-sale securities		1,974
Purchases of property and equipment	(30)	(90)
Net cash provided by investing activities	16,690	8,177
Cash Flows from Financing Activities:
Proceeds from exercise of common stock warrants and options and employee stock purchases	57	36
Payments on note payable	(70)	(63)
Payments on capital lease	(4)	(2)
Net cash used in financing activities	(17)	(29)
Net decrease in cash and cash equivalents	(973)	(4,707)
Cash and cash equivalents, beginning of period	80,667	26,586
Cash and cash equivalents, end of period	$ 79,694	$ 21,879