Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current Assets:
Cash and cash equivalents	$ 38,440	$ 58,856
Marketable securities - current	247,482	255,203
Accounts receivable	3,599	7,547
Prepaid expenses and other current assets	6,414	3,371
Total current assets	295,935	324,977
Marketable securities - noncurrent	9,868
Property and equipment, net	17,206	17,061
Operating lease right-of-use assets	20,663
Other assets	2,963	5,277
Total Assets	346,635	347,315
Current Liabilities:
Accounts payable	4,548	2,708
Accrued expenses	11,644	10,742
Current portion of lease liability	5,505
Current portion of deferred revenue	15,822	27,122
Total current liabilities	37,519	40,572
Deferred revenue, net of current portion	19,322	28,810
Long-term lease liability	14,142
Other long-term liabilities		13
Commitments and contingencies (Note 6)
Stockholders’ Equity:
Common stock, $0.0001 par value; 120,000,000 shares authorized; 48,877,297 and 45,224,480 shares issued and outstanding at September 30, 2019 and December 31, 2018, respectively	5	5
Additional paid-in capital	547,913	478,968
Accumulated other comprehensive income (loss)	336	(28)
Accumulated deficit	(272,602)	(201,025)
Total stockholders’ equity	275,652	277,920
Total Liabilities and Stockholders’ Equity	$ 346,635	$ 347,315