Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (66,572)	$ (33,970)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	186	39
Stock-based compensation	4,665	1,835
Non-cash expense in connection with equity issuance to MSK/MIT	1,331
Non-cash expense in connection with equity issuance to inventors	7,376
Foreign currency transactions	(66)	(11)
Changes in assets and liabilities:
Other current assets	(442)	635
Other assets	13	(57)
Accounts payable	1,305	3,015
Accrued liabilities and other	2,813	991
NET CASH USED IN OPERATING ACTIVITIES	(49,391)	(27,523)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of property and equipment	(28)	(201)
NET CASH USED IN INVESTING ACTIVITIES	(28)	(201)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from exercised stock options	370
NET CASH PROVIDED BY FINANCING ACTIVITIES	370
Effect of exchange rates on cash, cash equivalents and restricted cash	(28)	16
NET DECREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(49,077)	(27,708)
Cash, cash equivalents and restricted cash at the beginning of period	207,136	147,871
Cash, cash equivalents and restricted cash at the end of period	$ 158,059	120,163
SUPPLEMENTAL DISCLOSURE OF NON-CASH ACTIVITIES
Property and equipment purchases in accounts payable		23
Right-of-use assets obtained in exchange for lease obligations		$ 832