Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (13,152,000)	$ (22,946,000)	$ (16,829,000)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	2,813,000	516,000	92,000
Depreciation	1,111,000	965,000	712,000
Gain on disposal of equipment		(4,000)
Redeemable convertible preferred stock call option liability, net issued in connection with license and collaboration agreement			686,000
Change in fair value of redeemable convertible preferred stock call option liability, net		(314,000)	(387,000)
Changes in operating assets and liabilities:
Receivable from collaboration partner	(45,000,000)
Prepaid expenses and other current assets	(112,000)	(852,000)	(198,000)
Other long term assets	(24,000)	39,000
Accounts payable	(434,000)	1,301,000	(340,000)
Accrued liabilities	3,250,000	2,940,000	1,540,000
Other long term liabilities	(109,000)	454,000	(16,000)
Deferred revenue	30,801,000	(14,199,000)	28,398,000
Net cash (used in) provided by operating activities	(20,856,000)	(32,100,000)	13,658,000
Cash flow from investing activities
Purchases of investments	(16,060,000)
Purchases of property and equipment	(1,083,000)	(1,446,000)	(716,000)
Proceeds from sale of equipment		134,000
Decrease (increase) in restricted cash	30,000	60,000	(260,000)
Net cash used in investing activities	(17,113,000)	(1,252,000)	(976,000)
Cash flow from financing activities
Proceeds from issuance of common stock during IPO, net of issuance costs	(153,000)	55,780,000
Proceeds from exercise of stock options and employee stock purchase plan	506,000	352,000	165,000
Net cash provided by financing activities	61,501,000	101,969,000	29,055,000
Net increase in cash and cash equivalents	23,532,000	68,617,000	41,737,000
Cash and cash equivalents at beginning of period	112,265,000	43,648,000	1,911,000
Cash and cash equivalents at end of period	135,797,000	112,265,000	43,648,000
Non-cash investing and financing activities:
Unpaid financing-related costs included in period-end accounts payable	38,000
Unpaid financing-related costs included in period-end accrued liabilities		153,000
Vesting of early exercised options and restricted stock	272,000	177,000	104,000
Unpaid portion of property and equipment purchases included in period-end accounts payable	97,000	47,000	91,000
Unpaid portion of property and equipment purchases included in period-end accrued liabilities	6,000	14,000
Accretion of redeemable convertible preferred stock to redemption value		98,000	158,000
Accretion of dividends on redeemable convertible preferred stock		5,151,000	2,864,000
Follow-on Offering
Cash flow from financing activities
Issuance of common stock in connection with the 2016 follow-on offering, net of issuance costs	$ 61,148,000
Series A Redeemable Convertible Preferred Stock
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Change in fair value of redeemable convertible preferred stock call option liability, net			(15,000)
Cash flow from financing activities
Proceeds from issuance of preferred stock, net of issuance costs			18,994,000
Series A-1 Redeemable Convertible Preferred Stock
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Change in fair value of redeemable convertible preferred stock call option liability, net		(300,000)	(400,000)
Cash flow from financing activities
Proceeds from issuance of preferred stock, net of issuance costs			$ 9,896,000
Series B Redeemable Convertible Preferred Stock
Cash flow from financing activities
Proceeds from issuance of preferred stock, net of issuance costs		$ 45,837,000