Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Operating activities
Net loss	$ (21,318)	$ (16,746)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,703	996
Bad debt expense	68	54
Loss on disposal of property and equipment	12	0
Genzyme co-promotion fee amortization	(721)	(949)
Stock-based compensation	3,173	2,712
Conversion of accrued interest to long-term debt	192	0
Amortization and write-off of debt discount and issuance costs	119	23
Interest on debt balloon payment and prepayment penalty	206	39
Changes in operating assets and liabilities:
Accounts receivable	48	(589)
Supplies inventory	265	(279)
Prepaid expenses and current other assets	47	(349)
Other assets	(13)	(39)
Accounts payable	(805)	(4,637)
Accrued liabilities and deferred rent	712	(569)
Net cash used in operating activities	(16,312)	(20,333)
Investing activities
Purchases of property and equipment	(3,587)	(852)
Change in restricted cash	(2)	(533)
Net cash used in investing activities	(3,589)	(1,385)
Financing activities
Proceeds from the issuance of long-term debt, net of debt issuance costs	24,452	0
Proceeds from the issuance of common stock in a private placement, net of costs	0	37,258
Payment of long-term debt	(5,000)	0
Payment of end-of-term debt obligation and prepayment penalty	(288)	0
Proceeds from the exercise of common stock options and employee stock purchases	646	491
Net cash provided by financing activities	19,810	37,749
Net (decrease) increase in cash and cash equivalents	(91)	16,031
Cash and cash equivalents at beginning of period	39,084	35,014
Cash and cash equivalents at end of period	38,993	51,045
Supplementary cash flow information of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued liabilities	$ 42	$ 285