Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flow from operating activities
Net loss	$ (21,649)	$ (24,263)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	3,601	4,040
Depreciation	1,274	842
Amortization of premiums and discounts on marketable securities	102	(188)
Other non-cash items	(3,882)	808
Changes in operating assets and liabilities:
Related party collaboration receivable	3,759	2,213
Prepaid expenses and other current assets	323	(55)
Operating lease, right-of-use assets	951	734
Non-current assets	69	69
Accounts payable	2,543	745
Accrued expenses	(3,757)	(4,626)
Operating lease liabilities	(868)	(774)
Deferred revenue	(3,760)	(8,343)
Net cash used in operating activities	(21,294)	(28,798)
Cash flow from investing activities
Purchases of property and equipment	(375)	(1,507)
Proceeds from maturities of marketable securities	35,000	110,500
Net cash provided by investing activities	34,625	108,993
Cash flow from financing activities
Proceeds from the exercise of stock options	28	34
Net cash provided by financing activities	28	34
Net increase in cash and cash equivalents	13,359	80,229
Cash, cash equivalents, and restricted cash beginning of period	106,219	86,777
Cash, cash equivalents, and restricted cash end of period	119,578	167,006
Supplemental disclosure of cash and non-cash activities
Capital expenditures incurred but not yet paid	$ 17	$ 65