Condensed Statement of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities
Net loss	$ (12,657)	$ (5,474)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	5	2
Amortization of premium on investment securities	(123)	47
Stock‑based compensation expense	822	316
Accrued interest	(88)	125
Change in operating assets and liabilities:
Accounts receivable	552	0
Prepaid expenses and other current assets	75	(179)
Other long‑term assets	(85)	66
Accounts payable	1,039	(303)
Accrued expenses and other current liabilities	(385)	278
Deferred revenue	(449)	(449)
Other non‑current liabilities	4	(13)
Net cash used in operating activities	(11,290)	(5,584)
Investing activities
Purchase of investments	(72,778)	(8,228)
Proceeds from sales of investments	4,100	1,200
Proceeds from maturities of investments	22,900	14,615
Net cash used in investing activities	(45,778)	7,587
Financing activities
Proceeds from issuance of common stock, net of issuance costs	125,389	0
Net cash provided by financing activities	125,389	0
Net increase in cash and cash equivalents	68,321	2,003
Cash and cash equivalents at beginning of period	10,206	4,026
Cash and cash equivalents at end of period	78,527	6,029
Non-cash financing activities
Conversion of preferred stock to common stock	$ 109,327	$ 0