Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Operating activities
Net loss	$ (117,672,000)	$ (95,639,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	10,623,000	10,606,000
Depreciation expense	969,000	845,000
Non-cash lease expense	214,000	203,000
Amortization of premiums and accretion of discounts on available-for-sale securities, net	(1,414,000)	(422,000)
Changes in operating assets and liabilities:
Unbilled grant receivable	1,227,000	1,109,000
Prepaid expenses and other current and long-term assets, net	(113,000)	(1,257,000)
Insurance recovery receivable related to legal settlement	(1,628,000)
Accounts payable and accrued liabilities	7,367,000	2,101,000
Accrued legal settlement		10,000,000
Operating lease liability	(326,000)	(15,000)
Net cash used in operating activities	(100,753,000)	(72,469,000)
Investing activities
Purchases of available-for-sale securities	(27,671,000)	(95,288,000)
Maturities of available-for-sale securities	123,064,000	154,093,000
Purchases of property and equipment	(304,000)	(1,141,000)
Net cash provided by investing activities	95,089,000	57,664,000
Financing activities
Proceeds from exercise of common stock options and issuance of common stock under employee stock purchase plan.	493,000	654,000
Net cash provided by financing activities	493,000	654,000
Net decrease in cash, cash equivalents and restricted cash	(5,171,000)	(14,151,000)
Cash, cash equivalents and restricted cash, beginning of period	96,386,000	110,537,000
Cash, cash equivalents and restricted cash, end of period	$ 91,215,000	$ 96,386,000