Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities:
Net loss	$ (8,420)	$ (4,500)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	256	254
Stock-based compensation expense	414	35
Gain on sale of equipment		(15)
Change in fair value of redeemable convertible preferred stock call option liability, net		(314)
Change in operating assets and liabilities:
Prepaid expenses and other assets	37	(313)
Accounts payable	(115)	323
Accrued liabilities	(666)	71
Other long term liabilities	(11)	307
Deferred revenue	(3,550)	(3,550)
Net cash used in operating activities	(12,055)	(7,702)
Cash flows from investing activities:
Purchases of property and equipment	(562)	(562)
Proceeds from sale of equipment		134
Net cash used in investing activities	(562)	(428)
Cash flows from financing activities:
Proceeds from issuance of common stock		99
Payments of deferred offering costs	(153)
Net cash (used in) provided by financing activities	(153)	99
Net decrease in cash and cash equivalents	(12,770)	(8,031)
Cash and cash equivalents, beginning of period	112,265	43,648
Cash and cash equivalents, end of period	99,495	35,617
Supplemental disclosures of noncash investing and financing information
Vesting of early exercised options and restricted stock	71	29
Unpaid portion of property and equipment purchases included in period-end accounts payable	13	48
Unpaid portion of property and equipment purchases included in period-end accrued liabilities	$ 5	229
Accretion of dividends on redeemable convertible preferred stock		952
Accretion of redeemable convertible preferred stock to redemption value		$ 33