STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net loss	$ (35,374)	$ (30,310)	$ (15,143)
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Loss on disposal of property and equipment	25		26
Depreciation and amortization	1,206	783	655
Amortization of debt discount	80	40	12
Accretion of discount on short-term investments	1,186
Issuance of common stock in connection with a license agreement			198
Stock-based compensation expense	3,986	553	343
Deferred income taxes	8	8	10
Changes in operating assets and liabilities
Accounts receivable	1,131	(1,638)	(237)
Prepaid expenses and other current assets	(1,491)	(261)	(88)
Other assets	128	(344)	(78)
Accounts payable	2,944	660	366
Accrued liabilities	3,170	793	495
Deferred revenue	(6,130)	61,463	5,500
Net cash (used in)/provided by operating activities	(27,415)	31,802	(8,008)
Cash flows from investing activities:
Purchases of property and equipment	(1,594)	(1,663)	(732)
Purchases of short-term investments	(250,901)
Maturities of short-term investments	122,750
Increase in restricted cash	(817)
Net cash used in investing activities	(130,562)	(1,663)	(732)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	74,430	26,802
Proceeds from exercise of stock options	263	8	62
Proceeds from initial public offering, net of issuance costs	81,777
Proceeds from issuance of notes payable			3,359
Repayment of notes payable	(3,067)	(1,256)	(724)
Net cash provided by financing activities	153,403	25,554	2,697
Net increase/(decrease) in cash and cash equivalents	(4,574)	55,693	(6,043)
Cash and cash equivalents, beginning of year	64,396	8,703	14,746
Cash and cash equivalents, end of year	59,822	64,396	8,703
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	100	68
Accretion to redemption value and cumulative dividends on preferred stock	6,705	4,566	3,751
Convertible preferred stock liability recorded in connection with redeemable convertible preferred stock	1,509	908
Stock issuance costs in accounts payable and accrued liabilities	13	284
Convertible preferred stock warrants issued in connection with debt			82
Issuance costs in accounts payable and accrued liabilities			198
Convertible Preferred Stock Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability	1,114	13	(110)
Convertible Preferred Stock Warrant Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability	$ 602	$ 42	$ 43