CONDENSED STATEMENTS OF CASH FLOWS - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities
Net loss	$ (25,691,000)	$ (21,231,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,642,000	833,000
Bad debt expense	80,000	67,000
Genzyme co-promotion fee amortization	(1,423,000)	(1,794,000)
Stock-based compensation	4,186,000	2,402,000
Amortization of debt discount and issuance costs	34,000	81,000
Interest on debt balloon payment	59,000	60,000
Changes in operating assets and liabilities:
Accounts receivable	(550,000)	(104,000)
Supplies inventory	(288,000)	(783,000)
Prepaid expenses and other current assets	(1,878,000)	(552,000)
Other assets	(53,000)	(46,000)
Accounts payable	(1,831,000)	3,298,000
Accrued liabilities and deferred rent	1,578,000	(1,427,000)
Net cash used in operating activities	(24,135,000)	(19,196,000)
Investing activities
Purchases of property and equipment	(1,975,000)	(1,491,000)
Cash remitted for acquisition, net of cash received		(6,886,000)
Change in restricted cash	(533,000)	(100,000)
Net cash used in investing activities	(2,508,000)	(8,477,000)
Financing activities
Proceeds from issuance of common stock in a private placement, net of costs	37,258,000
Commissions and issuance costs relating to the initial public offering		(129,000)
Payment of deferred stock offering costs	(197,000)
Proceeds from the exercise of common stock options	684,000	584,000
Net cash provided by financing activities	37,745,000	455,000
Net increase (decrease) in cash and cash equivalents	11,102,000	(27,218,000)
Cash and cash equivalents at beginning of period	35,014,000	71,220,000
Cash and cash equivalents at end of period	46,116,000	44,002,000
Supplementary cash flow information of non cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued liabilities	1,173,000	$ 182,000
Unpaid deferred stock offering costs	$ 71,000