Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 173,712	$ 293,764
Marketable securities	460,883	0
Prepaid expenses and other current assets	7,700	7,273
Total current assets	642,295	301,037
Long-term investment—restricted	1,510	1,069
Property and equipment, net	5,274	13,453
Operating lease assets	522	0
Total assets	649,601	315,559
Current liabilities:
Accounts payable	14,770	8,308
Accrued expenses and other current liabilities	26,884	13,709
Operating lease liabilities	415	539
Notes payable to related party	187	187
Total current liabilities	42,256	22,743
Notes payable to related party, net of current portion	967	1,107
Operating lease liabilities, net of current portion	107	11,347
Other long-term liabilities	718	381
Total liabilities	44,048	35,578
Commitments and contingencies (Note 10)
Stockholders’ equity:
Preferred stock, $0.01 par value per share; 5,000,000 shares authorized; no shares issued or outstanding	0	0
Common stock, $0.01 par value per share; 125,000,000 shares authorized; 51,043,912 shares and 37,676,760 shares issued and outstanding as of September 30, 2019 and December 31, 2018, respectively	510	377
Additional paid-in capital	1,025,745	575,327
Accumulated other comprehensive income	61	0
Accumulated deficit	(420,763)	(295,723)
Total stockholders’ equity	605,553	279,981
Total liabilities and stockholders’ equity	$ 649,601	$ 315,559