Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (34,717)	$ (22,285)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	3,462	2,864
Non-cash interest expense	210	71
Depreciation and amortization expense	615	412
Purchase of premium on marketable securities	(3)	(12)
Amortization of premium on marketable securities	17	153
Changes in operating assets and liabilities:
Accounts receivable	39	(30)
Prepaid expenses and other current assets	(588)	877
Inventory	23	(15)
Accounts payable	2,922	(462)
Accrued expenses and deferred rent	2,312	(589)
Deferred revenue		(42)
Net cash used in operating activities	(25,708)	(19,058)
Cash flows from investing activities:
Purchases of property and equipment	(5,652)	(370)
Change in restricted cash		(1,500)
Purchases of marketable securities	(3,000)	(11,500)
Maturities of investments	35,200	50,000
Net cash provided by investing activities	26,548	36,630
Cash flows from financing activities:
Proceeds from issuance of notes payable	3,700
Proceeds from exercise of stock options	38	75
Proceeds from issuance of common stock pursuant to employee stock purchase plan	157	128
Proceeds from issuance of common stock offering, net	27,072
Payments of insurance costs financed by a third-party	(394)	(389)
Repayment of notes payable	(1,300)
Net cash provided by (used in) financing activities	29,273	(186)
Net increase in cash and cash equivalents	30,113	17,386
Cash and cash equivalents at beginning of period	32,936	30,784
Cash and cash equivalents at end of period	63,049	48,170
Supplemental disclosure of cash flow information:
Cash paid for interest	688
Supplemental disclosure of non-cash investing and financing activities:
Additions to property and equipment included in accounts payable at balance sheet dates	$ 1,269	$ 188