Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net income (loss)	$ 1,891	$ (33,452)	$ (34,156)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	178	366	410
Non-cash interest expense	26	108	36
Stock-based compensation expense	4,013	2,529	2,432
Change in fair value of warrant liability	(110)	(3,104)	123
(Gain) loss from change in fair value of contingent consideration	(1,100)	0	0
Loss on extinguishment of debt	221	0	459
Gain on sale of equipment	24	0	0
Expense related to issuance costs allocated to warrants measured at fair value	0	0	276
Changes in operating assets and liabilities:
Prepaid expenses and other assets	800	110	(259)
Restricted cash	84	0	0
Accounts payable	(742)	(1,212)	1,021
Accrued expenses	(1,936)	226	1,315
Deferred revenue	19	(100)	(964)
Due to related party	(698)	0	0
Net cash provided by (used in) operating activities	2,622	(34,529)	(29,307)
Investing activities
Cash acquired in acquisition	136	0	0
Sale (purchases) of property and equipment	325	(287)	(137)
Net cash provided by (used in) investing activities	461	(287)	(137)
Financing activities
Proceeds from issuance of notes payable, net of debt issuance costs	0	5,000	9,883
Payments on equipment financing and notes payable	(14,124)	(877)	(4,633)
Proceeds from issuance of common stock and common stock warrants, net of issuance costs	0	12,650	70,237
Proceeds from exercise of common stock options and common stock warrants	269	63	74
Proceeds from sale of common stock pursuant to ESPP	35	0	0
Net cash (used in) provided by financing activities	(13,820)	16,836	75,561
Net (decrease) increase in cash and cash equivalents	(10,737)	(17,980)	46,117
Cash and cash equivalents at beginning of period	36,079	54,059	7,942
Cash and cash equivalents at end of period	25,342	36,079	54,059
Supplemental non-cash investing and financing activities
Common stock issued in connection with the acquisition (Note 3)	13,525	0	0
Fair value of assets acquired in the acquisition, excluding cash	79,366	0	0
Fair value of liabilities assumed in the acquisition	19,777	0	0
Conversion of Series A and Series B preferred stock into 8,260,444 shares of common stock	0	0	56,678
Conversion of preferred stock warrants into common stock warrants	0	0	247
Issuance of warrants to purchase common stock	0	328	3,300
Supplemental cash flow information
Cash paid for interest	$ 663	$ 930	$ 335