Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 71,220	$ 14,002
Accounts receivable, net of allowance of $107 and $222 as of December 31, 2013 and 2012	1,143	569
Supplies inventory	2,567	1,050
Prepaid expenses and other current assets	1,477	710
Restricted cash		50
Total current assets	76,407	16,381
Property and equipment, net	2,952	2,446
Restricted cash	118	118
Other assets	153	122
Total assets	79,630	19,067
Current liabilities:
Accounts payable	5,294	1,888
Accrued liabilities	7,594	4,020
Deferred Genzyme co-promotion fee	2,500	2,500
Preferred stock liability		583
Total current liabilities	15,388	8,991
Long-term debt, net of current portion	4,899
Deferred rent, net of current portion	286	61
Deferred Genzyme co-promotion fee, net of current portion	2,614	5,114
Total liabilities	23,187	14,166
Commitments and contingencies (Note 8)
Convertible preferred stock, $0.001 par value; 0 and 59,147,999 shares authorized, 0 and 53,084,507 shares issued and outstanding as of December 31, 2013 and December 31, 2012; aggregate liquidation value of $0 and $65,835 as of December 31, 2013 and 2012		63,372
Stockholders' deficit:
Common stock, $0.001 par value; 125,000,000 and 77,000,000 shares authorized, 21,143,313 and 667,684 shares issued and outstanding as of December 31, 2013 and 2012	21	1
Additional paid-in capital	142,071	1,597
Preferred stock, $0.001 par value; 5,000,000 and 0 shares authorized, 0 shares issued and outstanding as of December 31, 2013 and 2012
Accumulated deficit	(85,649)	(60,069)
Total stockholders' equity (deficit)	56,443	(58,471)
Total liabilities, convertible preferred stock, and stockholders' equity (deficit)	$ 79,630	$ 19,067