CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (20,570)	$ (13,070)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	2,531	1,597
Accretion of discounts on marketable securities	(880)	(47)
Depreciation expense	38	10
Amortization on operating lease right-of-use asset	126
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,619)	(1,296)
Security deposits	(19)
Accounts payable	89	914
Accrued expenses	1,235	897
Due to related party	(38)	24
Operating lease liability	(88)
Net cash used in operating activities	(19,195)	(10,971)
Cash flows from investing activities
Sales and maturities of marketable securities	93,215
Purchases of marketable securities	(73,231)	(58,624)
Purchases of property, plant and equipment	(679)	(124)
Net cash provided by (used in) investing activities	19,305	(58,748)
Cash flows from financing activities
Proceeds from issuance of common stock in connection with IPO, net of offering costs		78,374
Proceeds from exercise of stock options	219
Net cash provided by financing activities	219	99,367
Net increase in cash and cash equivalents	329	29,648
Cash and cash equivalents at the beginning of the period	10,271	8,663
Cash and cash equivalents at the end of the period	10,600	38,311
Supplemental disclosure of noncash investing and financing activities:
Construction in process purchases payable or accrued at period end	441
Change in unrealized gain (loss) on marketable securities	$ 44	(7)
Conversion of preferred stock into common stock		36,501
Series C Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of preferred stock, net of issuance costs		$ 20,993