CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net loss	$ (27,141)	$ (18,672)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	85	84
Stock-based compensation expense	5,035	1,876
Noncash lease expense	73	62
Noncash lease liability	82	95
Amortization of discount on investments		1
Changes in operating assets and liabilities:
Prepaid expenses and other current and non-current assets and federal contract revenue receivables	366	258
Accounts payable, accrued expenses and other long term-liabilities	5,325	2,259
Net cash used in operating activities	(16,175)	(14,037)
Cash flows from investing activities
Maturities of short-term investments	1,474	1,737
Purchases of short-term investments		(8,433)
Deposit on property and equipment	(404)
Purchases of property and equipment	(28)	(1)
Net cash provided by (used in) investing activities	1,042	(6,697)
Cash flows from financing activities
Proceeds from exercise of stock options	244	312
Financing costs	(148)	(195)
Net cash provided by financing activities	96	117
Net decrease in cash and cash equivalents	(15,037)	(20,617)
Cash and cash equivalents-beginning of period	138,509	90,943
Cash and cash equivalents-end of period	$ 123,472	$ 70,326