Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities
Net loss	$ (8,114)	$ (7,852)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	112	101
Stock-based compensation	665	710
Amortization of debt issuance cost	16	34
Net accretion/amortization on investments	(42)	(54)
Changes in assets and liabilities:
Accounts receivable	(922)	500
Inventories	320	(91)
Prepaid expenses and other assets	(419)	68
Accounts payable	(948)	(292)
Accrued and other liabilities	220	827
Contract research liabilities	4	(121)
Deferred revenue	(9)	(215)
Total adjustments	(1,003)	1,467
Net cash used in operating activities	(9,117)	(6,385)
Cash flows from investing activities
Purchases of property and equipment	(6)	(12)
Purchases of available-for-sale securities		(5,132)
Proceeds from maturities of available-for-sale securities	7,234	10,479
Net cash provided by investing activities	7,228	5,335
Cash flows from financing activities
Payments on equipment financing obligations	(3)	(6)
Net proceeds from issuances of common stock	760	1,326
Net cash provided by financing activities	757	1,320
Net (decrease) increase in cash and cash equivalents	(1,132)	270
Cash and cash equivalents, beginning of the period	5,404	3,583
Cash and cash equivalents, end of the period	$ 4,272	$ 3,853