Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (10,538)	$ (8,048)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	1,363	180
Change in fair value of preferred stock warrant liabilities		1,326
Depreciation and amortization	52	13
Other	13	9
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(85)	229
Accrued liabilities	(1,451)	(276)
Accounts payable	1,415	1,581
Net cash used in operating activities	(9,231)	(4,986)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(179)	(12)
Change in restricted cash	25	(50)
Purchase of short-term investments		(9)
Net cash used in investing activities	(154)	(71)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from exercise of common stock options	96	15
Payment of deferred offering costs	(15)	(71)
Proceeds from early exercise of stock options		13
Net cash provided by (used in) financing activities	81	(43)
Effect of exchange rate changes on cash and cash equivalents	(5)
NET DECREASE IN CASH AND CASH EQUIVALENTS	(9,309)	(5,100)
CASH AND CASH EQUIVALENTS - Beginning of period	150,180	29,154
CASH AND CASH EQUIVALENTS - End of period	140,871	24,054
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes	0	0
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Change in redemption value of redeemable convertible preferred stock		(11,005)
Deferred offering costs included in accounts payable and accrued liabilities		$ 944
Property and equipment purchases included in accounts payable and accrued liabilities	$ 20