Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current Assets:
Cash and cash equivalents	$ 49,140	$ 58,856
Marketable securities - current	221,959	255,203
Accounts receivable	4,188	7,547
Proceeds due from at-the-market offerings	27,140
Prepaid expenses and other current assets	4,710	3,371
Total current assets	307,137	324,977
Marketable securities - noncurrent	4,739
Property and equipment, net	16,694	17,061
Operating lease right-of-use assets	20,923
Other assets	2,948	5,277
Total Assets	352,441	347,315
Current Liabilities:
Accounts payable	1,848	2,708
Accrued expenses	11,898	10,742
Current portion of lease liability	5,065
Current portion of deferred revenue	19,652	27,122
Total current liabilities	38,463	40,572
Deferred revenue, net of current portion	22,508	28,810
Long-term lease liability	14,604
Other long-term liabilities		13
Commitments and contingencies
Stockholders’ Equity:
Common stock, $0.0001 par value; 120,000,000 shares authorized; 47,714,575 and 45,224,480 shares issued and outstanding at June 30, 2019 and December 31, 2018, respectively	5	5
Additional paid-in capital	525,574	478,968
Accumulated other comprehensive income (loss)	255	(28)
Accumulated deficit	(248,968)	(201,025)
Total stockholders’ equity	276,866	277,920
Total Liabilities and Stockholders’ Equity	$ 352,441	$ 347,315