Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Cash flows from operating activities:
Net loss	$ (28,675)	$ (11,556)	$ (9,128)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and write-offs of fixed assets	411	230	80
Write-offs of patents			95
Stock-based compensation	4,255	2,249	2,631
Amortization of deferred license fees	(4,143)	(5,318)	(4,733)
Warrant licensing expense	107	107	1,779
Changes in assets and liabilities:
Prepaid expenses and other current assets	(788)	238	(294)
Accounts payable	(263)	873	(528)
Accrued expenses	1,251	667	(81)
Accrued interest	21	20	1
Third party royalty obligation		(988)	(2,462)
Net cash used in operating activities	(27,824)	(13,478)	(12,640)
Cash flows from investing activities:
Purchases of marketable securities	(87,017)	(24,498)	(18,879)
Maturities of marketable securities	46,085	10,241	16,474
Purchases of property and equipment	(2,115)	(626)	(462)
Net cash used in investing activities	(43,047)	(14,883)	(2,867)
Cash flows from financing activities:
Proceeds from Genzyme Collaboration, License and Purchase Agreement		1,896	16,650
Proceeds from sale of common stock to Genzyme			6,000
Proceeds from sale of common stock, net of issuance costs	27,215	82,327	8,062
Proceeds from exercise of common stock options and stock purchase plan	685	466	732
Decrease in restricted cash		500	100
Payment for repurchase of stock options			(50)
Proceeds from long term debt			1,000
Net cash provided by financing activities	27,900	85,189	32,494
Net increase (decrease) in cash and cash equivalents	(42,971)	56,828	16,987
Cash and cash equivalents, beginning of period	78,752	21,924	4,937
Cash and cash equivalents, end of period	35,781	78,752	21,924
Supplemental disclosure of non-cash investing and financing activities:
Unrealized gain (loss) on available-for-sale investments	(15)	(2)	1
Retirement of 85,550 treasury shares of common stock			97
Issuance of 27,872, 15,460, and 24,430 shares of common stock to fund the Company's 401(k) matching contribution for 2010, 2009, and 2008, respectively	$ 169	$ 65	$ 32