CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (73,708)	$ (56,322)
Adjustments to reconcile net loss to net cash from operating activities:
Stock-based compensation expense	4,961	4,883
Non-cash interest expense	252	1
Fair value of Series B redeemable convertible preferred stock earned in connection with sponsored research agreement		253
Fair value of common stock earned in connection with license agreement	2,660
Depreciation expense	3,080	1,814
Accretion of investments	(40)	(925)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,098	(3,043)
Operating right-of-use assets	959
Other non-current assets	(2)	102
Accounts payable	(168)	855
Accrued expenses	10,715	5,960
Deferred revenue	8,725	55,762
Deferred rent		3,577
Operating lease liabilities	10,977
Net cash provided by (used in) operating activities	(29,491)	12,917
Cash flows from investing activities:
Purchases of property and equipment	(19,891)	(3,521)
Purchases of investments		(108,081)
Maturities of investments	73,063	107,000
Net cash provided by (used in) investing activities	53,172	(4,602)
Cash flows from financing activities:
Proceeds from long-term debt, net of issuance costs	15,000	9,567
Proceeds from exercise of common stock options	142	254
Payment of initial public offering costs	(1,167)
Net cash provided by financing activities	13,975	9,821
Increase in cash, cash equivalents and restricted cash	37,656	18,136
Cash, cash equivalents and restricted cash, beginning of period	14,853	18,460
Cash, cash equivalents and restricted cash, end of period	52,509	36,596
Supplemental disclosures:
Cash paid for interest	834
Non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses	1,134	1,346
Deferred offering costs included in accrued expenses	493
Deferred financing costs for long-term debt included in accounts payable and accrued expenses		83
Accretion of redeemable convertible preferred stock to redemption value	10,249	$ 9,984
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	$ 23,186