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Consolidated Statements of Cash Flows - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Cash flows from operating activities:    
Net loss $ (52,412) $ (75,305)
Adjustments to reconcile net loss to net cash used in operating activities:    
Stock-based compensation expense 31,960 4,456
Depreciation expense 3,549 2,845
Net amortization of premiums and discounts on investments (416) (2,495)
Loss on sale of equipment   56
Changes in assets and liabilities:    
Prepaid expenses and other current assets (4,665) (2,123)
Lease assets and liabilities, net 571 1,233
Accounts payable (410) 3,000
Accrued expenses and other liabilities 2,010 2,200
Accounts receivable (75,000)  
Contract asset (7,654)  
Other assets (22)  
Net cash used in operating activities (102,489) (66,133)
Cash flows from investing activities:    
Purchases of property and equipment (1,931) (8,002)
Proceeds from sale of equipment   20
Purchases of investments (266,455) (553,517)
Proceeds from maturities of investments 350,058 242,475
Net cash provided by (used in) investing activities 81,672 (319,024)
Cash flows from financing activities:    
Proceeds from issuance of convertible preferred stock, net of issuance costs   4,993
Proceeds from issuance of common stock upon exercise of stock options 1,216 613
Proceeds from issuance of common stock upon initial public offering 427,800  
Offering costs paid (2,507)  
Net cash provided by financing activities 426,509 5,606
Net increase (decrease) in cash, cash equivalents and restricted cash 405,692 (379,551)
Cash, cash equivalents and restricted cash at beginning of period 42,832 422,383
Cash, cash equivalents and restricted cash at end of period 448,524 42,832
Supplemental disclosure of non-cash investing and financing activities:    
Property and equipment additions included in accounts payable and accrued expenses 519 745
Reclassification of restricted stock liability to additional paid-in capital 153 400
Extinguishment upon modification of Series C preferred stock (177,789)  
Conversion of preferred stock into common stock upon initial public offering 715,508  
Right of use asset obtained in exchange for lease obligation $ 819 $ 24,936