Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (34,509)	$ (22,663)	$ (22,110)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	416	425	601
Stock-based compensation	2,655	2,660	3,080
Inventory write-off	687	303	1,227
Amortization of debt issuance cost	104	100	38
Loss on debt extinguishment	9
Net accretion/amortization on investments	(216)	(278)	(351)
Realized gain from sale of marketable equity security, net of tax		(117)
Changes in assets and liabilities:
Accounts receivable	1,068	(100)	227
Inventories	(552)	(579)	(1,369)
Prepaid expenses and other assets	656	(2,056)	854
Accounts payable	800	265	285
Accrued liabilities	1,419	1,385	1,200
Contract research liability	208	217	29
Deferred revenue	(38)	(395)	1,729
Total adjustments	7,216	1,830	7,550
Net cash used in operating activities	(27,293)	(20,833)	(14,560)
Cash flows from investing activities
Purchases of property and equipment	(147)	(225)	(204)
Purchases of available-for-sale securities	(24,400)	(34,040)	(31,472)
Proceeds from maturities of available-for-sale securities	30,584	40,619	16,294
Proceeds from sales of short-term investment		178
Net cash provided by (used in) investing activities	6,037	6,532	(15,382)
Cash flows from financing activities
Payments on equipment financing obligations	(19)	(21)	(17)
Net proceeds from issuances of common stock upon exercise of stock options, and purchases of ESPP shares	410	1,175	399
Net proceeds from issuances of common stock in connection with equity financings	23,745	14,290	4,618
Payment of additional issuance cost for term loan	(173)	(240)
Payment of final payment for term loan	(886)
Net proceeds from issuance of term loan			19,786
Net cash provided by financing activities	23,077	15,204	24,786
Net increase (decrease) in cash and cash equivalents	1,821	903	(5,156)
Cash and cash equivalents at beginning of year	3,583	2,680	7,836
Cash and cash equivalents at end of year	5,404	3,583	2,680
Supplemental disclosure of cash flow information
Cash paid for interest	$ 1,611	$ 1,595	$ 691