Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current Assets:
Cash and cash equivalents	$ 361,687	$ 57,226
Marketable securities	75,117	222,500
Accounts receivable	3,864	4,620
Prepaid expenses and other current assets	5,699	5,135
Total current assets	446,367	289,481
Marketable securities - noncurrent		4,746
Property and equipment, net	16,402	17,996
Operating lease right-of-use assets	23,469	19,137
Other assets	4,592	2,920
Total Assets	490,830	334,280
Current Liabilities:
Accounts payable	7	3,941
Accrued expenses	15,374	13,273
Current portion of operating lease liability	5,834	5,745
Current portion of deferred revenue	37,927	12,674
Total current liabilities	59,142	35,633
Deferred revenue, net of current portion	62,752	16,136
Long-term operating lease liability	17,349	12,630
Commitments and contingencies (Note 6)
Stockholders’ Equity:
Common stock, $0.0001 par value; 120,000,000 shares authorized; 58,724,238 and 50,198,044 shares issued and outstanding at June 30, 2020 and December 31, 2019, respectively	6	5
Additional paid-in capital	716,503	570,493
Accumulated other comprehensive income	155	261
Accumulated deficit	(365,077)	(300,878)
Total stockholders’ equity	351,587	269,881
Total Liabilities and Stockholders’ Equity	$ 490,830	$ 334,280