Consolidated Balance Sheets - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 190,943,000	$ 306,235,000
Marketable securities	731,833,000	212,613,000
Prepaid expenses and other current assets	21,919,000	6,227,000
Total current assets	944,695,000	525,075,000
Property and equipment, net	5,643,000	4,013,000
Restricted cash	2,367,000	849,000
Total assets	952,705,000	529,937,000
Current liabilities:
Accounts payable	34,036,000	9,350,000
Accrued expenses	51,994,000	42,601,000
Total current liabilities	86,030,000	51,951,000
Other liabilities	3,704,000	2,511,000
Total liabilities	89,734,000	54,462,000
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 5,000,000 shares authorized at December 31, 2018 and 2017; no shares issued or outstanding at December 31, 2018 and 2017
Common stock, $0.0001 par value per share; 120,000,000 shares authorized at December 31, 2018 and 2017; 46,891,296 and 42,003,894 shares issued at December 31, 2018 and 2017, respectively; 46,888,263 and 42,002,934 shares outstanding at December 31, 2018 and 2017, respectively	5,000	5,000
Treasury stock, at cost, 3,033 and 960 shares at December 31, 2018 and December 31, 2017, respectively	(211,000)	(113,000)
Additional paid-in capital	1,827,021,000	1,066,059,000
Accumulated deficit	(963,329,000)	(590,447,000)
Accumulated other comprehensive loss	(515,000)	(29,000)
Total stockholders’ equity	862,971,000	475,475,000
Total liabilities and stockholders’ equity	$ 952,705,000	$ 529,937,000