CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (7,576)	$ (4,981)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	24	27
Amortization of premium (discount) on marketable securities	(3)	26
Amortization of debt discount	60
Non-cash stock compensation	543	594
Changes in operating assets and liabilities:
Prepaid expenses and other assets	280	(115)
Accounts payable and accrued expenses	(1,653)	(843)
Deferred revenue		(1,140)
Net cash used in operating activities	(8,325)	(6,432)
Cash flows from investing activities:
Purchases of marketable securities	(1)	(13,386)
Proceeds from sale or maturity of marketable securities	6,556	10,498
Net cash provided by (used in) investing activities	6,555	(2,888)
Cash flows from financing activities:
Proceeds from notes payable and warrants, net	14,740
Proceeds from stock offering, net		15,185
Proceeds from stock option exercises and employee stock plan purchases		114
Net cash provided by financing activities	14,740	15,299
Net increase in cash and cash equivalents	12,970	5,979
Cash and cash equivalents, beginning of period	15,267	13,983
Cash and cash equivalents, end of period	$ 28,237	$ 19,962