Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (45,290)	$ (31,390)	$ (20,610)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,465	691	584
Stock-based compensation expense	4,168	1,653	578
Noncash interest expense	354	112	44
Change in fair value of warrants		1	530
Loss on disposal of asset			6
Deferred rent	(119)	5	(5)
Changes in operating assets and liabilities:
Accounts receivable	(168)	(201)
Prepaid expenses and other assets	190	(1,217)	(138)
Inventory	(568)	(115)
Accounts payable	177	(208)	372
Accrued expenses and other liabilities	260	2,405	586
Deferred revenue	2,066	80
Net cash used in operating activities	(37,465)	(28,184)	(18,053)
Investing activities
Purchases of property and equipment	(7,974)	(2,084)	(513)
Decrease in restricted cash	80		80
Net cash used in investing activities	(7,894)	(2,084)	(433)
Financing activities
Proceeds from issuance of common stock in public offering, net of offering costs	33,677	58,089
Proceeds from issuance of redeemable convertible preferred stock, net			39,768
Proceeds from issuance of common stock and stock options exercises, net	1,804	153	55
Proceeds from issuance of note payable, net	10,000	19,714
Repayments of notes payable	(309)	(4,037)	(848)
Net cash provided by financing activities	45,172	73,919	38,975
Net (decrease) increase in cash and cash equivalents	(187)	43,651	20,489
Cash and cash equivalents at beginning of period	73,849	30,198	9,709
Cash and cash equivalents at end of period	73,662	73,849	30,198
Supplemental disclosures of cash flow information
Cash paid for interest	1,506	515	345
Supplemental disclosures of noncash investing and financing activities
Accrued property and equipment	247	128
Leasehold improvements paid by landlord	1,268	121
Public offering costs unpaid at year end	$ 420
Accretion of Series A-1, A-2, B, C, D and E redeemable convertible preferred stock to redemption value		4,570	$ 6,908
Conversion of redeemable and convertible preferred stock to common stock		117,383
Conversion of preferred warrants to common stock		$ 1,226