Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (16,023)	$ (10,840)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	1,705	1,349
Non-cash interest expense	110	35
Depreciation and amortization expense	261	205
Purchase of premium on marketable securities	(3)
Amortization of premium on marketable securities	11	104
Changes in operating assets and liabilities:
Accounts receivable	6	(34)
Prepaid expenses and other current assets	(1,167)	305
Inventory	17	7
Accounts payable	659	79
Accrued expenses and deferred rent	(153)	(424)
Deferred revenue		(42)
Net cash used in operating activities	(14,577)	(9,256)
Cash flows from investing activities:
Purchases of property and equipment	(1,654)	(127)
Purchases of marketable securities	(3,000)
Maturities of investments	12,500	25,000
Net cash provided by investing activities	7,846	24,873
Cash flows from financing activities:
Proceeds from issuance of notes payable	3,700
Proceeds from exercise of stock options	2	5
Proceeds from issuance of common stock offering, net	26,272
Payments of insurance costs financed by a third-party	(197)	(194)
Repayment of notes payable	(1,300)
Net cash provided by (used in) financing activities	28,477	(189)
Net increase in cash and cash equivalents	21,746	15,428
Cash and cash equivalents at beginning of period	32,936	30,784
Cash and cash equivalents at end of period	54,682	46,212
Supplemental disclosure of non-cash investing and financing activities:
Additions to property and equipment included in accounts payable at balance sheet dates	987	$ 109
Public offering costs included in accounts payable and accrued expenses at balance sheet dates	$ 55