Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (43,755)	$ (81,714)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,770	121
Shares issued in connection with Amgen Agreement		1,353
Acquisition of technology in connection with Amgen Agreement		5,000
Issuance date fair-value of anti-dilution liability		1,639
Change in fair value of preferred stock tranche liability		62,150
Change in fair value of anti-dilution right liability		5,765
Net amortization of premiums and discounts on short-term investments	(104)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(507)	(1,059)
Accounts payable	(426)	1,313
Accrued expenses and other current liabilities	7,395	807
Net cash used in operating activities	(35,627)	(4,625)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of short-term marketable securities	(71,513)
Acquisition of technology in connection with Amgen Agreement		(5,000)
Net cash used in investing activities	(71,513)	(5,000)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock in initial public offering, net of issuance costs and underwriting fees	95,452
Proceeds from the early exercise of stock options in exchange for restricted common stock	321
Proceeds from the exercise of stock options	130	8
Proceeds from the issuance of common stock pursuant to employee stock purchase plan purchases	85
Payment of initial public offering costs		(52)
Payment of preferred stock issuance costs		(449)
Net cash provided by financing activities	95,988	85,007
Net (decrease) increase in cash, cash equivalents and restricted cash	(11,152)	75,382
Cash and restricted cash at the beginning of the year	76,000	618
Cash, cash equivalents and restricted cash at the end of the year	64,848	76,000
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Conversion of convertible preferred stock into common stock	124,728
Net unrealized loss on marketable securities	$ (6)
Accretion of redeemable convertible preferred stock to redemption value		520
Issuance date fair value of preferred stock tranche obligation		7,350
Deferred offering costs included in accounts payable and accrued expenses and other current liabilities		309
Shares issued in connection with Amgen Agreement		(1,353)
Settlement of future purchase obligation		32,750
Extinguishment of call option liability		36,750
Series A Preferred Stock
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of redeemable convertible preferred stock		40,000
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Issuance of Series A redeemable convertible preferred stock in settlement of anti dilution right liability		7,404
Series B Preferred Stock
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of redeemable convertible preferred stock		$ 45,500