Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (21,443)	$ (11,032)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	2,609	698
Depreciation and amortization	430	124
Noncash lease expense	109
Accretion of investment securities and purchase discounts, net of amortization of purchase premiums	(689)
Other	2	76
Increase (decrease) in cash resulting from changes in:
Prepaid expenses and other current assets	24	(1,275)
Accounts payable and accrued expenses	(92)	3,652
Operating lease liability	(277)
Net cash used in operating activities	(19,327)	(7,757)
Cash flows from investing activities:
Purchases of investment securities	(52,664)
Maturities of investment securities	87,170
Purchases of property and equipment	(464)	(526)
Net cash provided by (used in) investing activities	34,042	(526)
Financing activities:
Proceeds from issuance of convertible preferred stock, net		63,266
Proceeds from exercise of stock options	87	393
Repurchase of unvested shares	(59)
Payment of initial public offering costs		(660)
Net cash provided by financing activities	28	62,999
Net change in cash, cash equivalents and restricted cash	14,743	54,716
Cash, cash equivalents and restricted cash at beginning of period	45,473	14,192
Cash, cash equivalents and restricted cash at end of period	60,216	68,908
Noncash investing and financing activities:
Purchase of shares pursuant to Employee Stock Purchase Plan	379
Change in unvested stock liability	$ (71)	(7)
Amounts accrued for purchases of property and equipment		52
Tenant improvement allowance		954
Accrued but unpaid preferred stock issuance and initial public offering costs		$ 1,046