Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net loss	$ (52,546)	$ (36,135)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,096	4,846
Depreciation and amortization expense	113	125
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	191	175
Accounts payable	3,379	(63)
Accrued expense	(1,147)	632
Accrued interest, net of interest received on maturity of investments	485	(86)
Net cash used in operating activities	(43,429)	(30,506)
Cash flows from investing activities:
Purchases of marketable securities	(93,722)	(58,647)
Sales and maturities of marketable securities	60,302	109,421
Purchases of property and equipment, net of disposals	(44)	(314)
Net cash provided by (used in) investing activities	(33,464)	50,460
Cash flows from financing activities:
Proceeds from issuances of stock, excluding related parties, net of transaction costs	66,616
Proceeds from the exercise of common stock options, net of transaction costs	478
Net cash provided by financing activities	67,094
Net increase (decrease) in cash and cash equivalents	(9,799)	19,954
Cash and cash equivalents at beginning of period	54,004	46,697
Cash and cash equivalents at end of period	$ 44,205	$ 66,651