CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities
Net loss	$ (23,221)	$ (6,193)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	1,272	88
(Amortization of premiums) and accretion of discounts on investment securities, net	(1,220)
Depreciation	194	91
Change in fair value of purchase right liability		14
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,894)	(120)
Prepaid expenses and other current assets—related parties	28	(88)
Accounts payable and accrued liabilities	(5)	1,172
Accounts payable and accrued liabilities—related parties	151	(275)
Deferred rent - related parties		44
Operating lease right-of-use assets and liabilities, net	74
Operating lease right-of-use assets and liabilities, net—related parties	40
Other assets	50
Net cash used in operating activities	(24,531)	(5,267)
Cash flows from investing activities
Purchases of investment securities	(169,446)
Proceeds from maturities of investment securities	29,250
Purchases of property and equipment	(440)	(337)
Net cash used in investing activities	(140,636)	(337)
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock and Series C preferred stock purchase right liability, net of issuance costs		26,255
Proceeds from exercise of common stock options and stock issuances under the employee stock purchase plan	345	1
Net cash provided by financing activities	345	26,256
Net (decrease) increase in cash and cash equivalents	(164,822)	20,652
Cash and cash equivalents
Beginning of period	188,356	3,661
End of period	$ 23,534	$ 24,313