CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (8,262)	$ (22,078)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	140	115
Stock-based compensation	2,975	2,867
Accretion/amortization of investments, net	(8)	(1,028)
Non-cash interest expense	0	164
Changes in operating assets and liabilities:
Receivable from collaborative partners	(5,000)	0
Prepaid expenses and other assets	233	(1,215)
Accounts payable and other liabilities	(5,190)	5,076
Net cash used in operating activities	(15,112)	(16,099)
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of investments	(74,013)	(79,763)
Sales and maturities of investments	84,851	89,600
Purchases of property and equipment	(87)	(180)
Net cash provided by investing activities	10,751	9,657
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, upon the exercise of stock options	36	574
Payments on notes payable	(1,375)	(1,875)
Net cash used in financing activities	(1,339)	(1,301)
Net decrease in cash, cash equivalents, and restricted cash	(5,700)	(7,743)
Cash, cash equivalents and restricted cash, beginning of period	171,077	113,656
Cash, cash equivalents and restricted cash, end of period	165,377	105,913
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Interest paid	4	172
Non-cash investing and financing activities:
Amounts accrued for property and equipment	$ 86	$ 233