CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Operating Activities:
Net loss	$ (84,019)	$ (59,212)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	2,200	859
Loss on disposal of property and equipment	5
Stock-based compensation	8,204	3,125
Change in preferred stock warrant liability		199
Accretion of discount on issued term debt	332	476
Accretion of investment discount, net	114
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,733	(1,817)
Operating lease right-of-use assets	1,817	2,201
Other long-term assets	(208)	(137)
Accounts payable	3,727	(3,654)
Accrued expenses and other liabilities	(2,694)	6,233
Operating lease liabilities	(1,355)	(1,959)
Net cash used in operating activities	(70,144)	(53,686)
Investing Activities:
Purchases of property and equipment	(2,056)	(11,955)
Purchases of short-term investments		(19,884)
Proceeds from maturities of short-term investments	187,500	37,500
Net cash provided by investing activities	185,444	5,661
Financing Activities:
Net proceeds from stock option exercises	427	183
Payment of deferred offering costs		(787)
Payment of debt issuance costs		(325)
Net proceeds from CIRM grant awards		4,163
Net cash provided by financing activities	427	107,374
Net increase in cash and cash equivalents	115,727	59,349
Cash and cash equivalents at beginning of period	83,966	87,784
Cash and cash equivalents at end of period	199,693	147,133
Non-cash operating, investing and financing activities:
Purchases of property and equipment in accounts payable and accrued liabilities	109	2,186
Tenant improvement receivable from landlord		1,314
Deferred offering costs incurred but not yet paid		1,092
Supplemental disclosure of cash flow information:
Interest paid	$ 1,356	1,660
Series D Convertible Preferred Stock
Financing Activities:
Issuance of Series D financing, net of issuance costs		$ 104,140