Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Operating activities
Net loss	$ (26,955)	$ (25,691)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,602	1,642
Bad debt expense	68	80
Loss on disposal of property and equipment	12	0
Genzyme co-promotion fee amortization	(948)	(1,423)
Stock-based compensation	4,745	4,186
Conversion of accrued interest to long-term debt	385	0
Amortization and write-off of debt discount and issuance costs	146	34
Interest on debt balloon payment and prepayment penalty	206	59
Changes in operating assets and liabilities:
Accounts receivable	(2,877)	(550)
Supplies inventory	351	(288)
Prepaid expenses and current other assets	37	(1,878)
Other assets	(44)	(53)
Accounts payable	(387)	(1,831)
Accrued liabilities and deferred rent	(1,091)	1,578
Net cash used in operating activities	(23,750)	(24,135)
Investing activities
Purchases of property and equipment	(3,760)	(1,975)
Change in restricted cash	(2)	(533)
Net cash used in investing activities	(3,762)	(2,508)
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 deferred stock offering costs	0	(197)
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	963	684
Net cash provided by financing activities	20,127	37,745
Net (decrease) increase in cash and cash equivalents	(7,385)	11,102
Cash and cash equivalents at beginning of period	39,084	35,014
Cash and cash equivalents at end of period	31,699	46,116
Supplementary cash flow information of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued liabilities	0	1,173
Unpaid deferred stock offering	$ 0	$ 71