CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities
Net loss	$ (23,078)	$ (15,653)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	208	43
Lease incentive	(9)	(8)
Stock-based compensation expense	1,821	1,101
Changes in operating assets and liabilities:
Accounts receivable	(1,220)
Inventories	(179)
Prepaid expenses and other assets	(148)	365
Accounts payable	(2,313)	2,651
Accrued expenses	(2,521)	(15)
Deferred revenue	3,751
Net cash used in operating activities	(23,688)	(11,516)
Investing activities
Purchases of property and equipment	(29)
Net cash used in investing activities	(29)
Financing activities
Proceeds from issuances of common stock from public offerings, net of issuance costs of $526		51,174
Proceeds from issuances of common stock from employee stock purchase plans	673
Repayment of term note	(666)	(667)
Proceeds from the exercise of stock options	94	42
Payments made for employee restricted stock tax withholdings	(51)
Net cash provided by financing activities	50	50,549
Net (decrease) increase in cash and cash equivalents	(23,667)	39,033
Cash and cash equivalents at beginning of period	153,225	95,697
Cash and cash equivalents at end of period	129,558	134,730
Supplemental disclosure of cash flow information
Cash paid for offering costs		384
Cash paid for interest	49	83
Supplemental disclosure of non-cash activities
Offering costs in accrued expenses	112	$ 128
Acquisition of property and equipment in accrued expenses	$ 99