CONDENSED STATEMENTS OF CASH FLOWS (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Operating activities
Net loss	$ (19,688,000)	$ (13,812,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	717,000	526,000
Bad debt expense	184,000	144,000
Genzyme co-promotion fee amortization	(1,875,000)	(1,761,000)
Stock-based compensation	851,000	458,000
Equity-based compensation		203,000
Amortization of debt discount and issuance costs	28,000
Interest on debt balloon payment	21,000
Change in value of preferred stock liability	2,070,000
Change in value of preferred stock warrant liability	77,000
Changes in operating assets and liabilities:
Accounts receivables	(329,000)	(482,000)
Supplies inventory	(342,000)	(488,000)
Prepaid expenses and current other assets	(2,183,000)	(287,000)
Other assets	32,000	(58,000)
Accounts payable	3,813,000	747,000
Accrued liabilities and deferred rent	763,000	1,531,000
Deferred Genzyme co-promotion fee		10,000,000
Net cash used in operating activities	(15,861,000)	(3,279,000)
Investing activities
Purchases of property and equipment	(1,061,000)	(912,000)
Change in restricted cash	50,000
Net cash used in investing activities	(1,011,000)	(912,000)
Financing activities
Proceeds from the issuance of long-term debt, net of debt issuance costs	4,877,000
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	12,945,000
Proceeds from the exercise of common stock options	474,000	66,000
Net cash provided by financing activities	18,296,000	66,000
Net increase (decrease) in cash and cash equivalents	1,424,000	(4,125,000)
Cash and cash equivalents at beginning of period	14,002,000	7,566,000
Cash and cash equivalents at end of period	$ 15,426,000	$ 3,441,000