Condensed Statement of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net loss	$ (33,852)	$ (19,800)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	10	5
Amortization of premium on investment securities	(551)	73
Stock-based compensation expense	2,564	1,217
Disposal of equipment		19
Change in operating assets and liabilities:
Accounts receivable	786	(460)
Prepaid expenses and other current assets	1,763	(1,143)
Other long-term assets	(354)	66
Accounts payable	240	2,001
Accrued expenses and other current liabilities	1,819	1,320
Deferred revenue	(8,531)	(1,347)
Other non-current liabilities	(10)	(16)
Net cash used in operating activities	(36,116)	(18,065)
Investing activities
Purchase of property plant and equipment	(118)	(15)
Purchase of investments	(109,952)	(59,171)
Proceeds from sales of investments	5,100	6,000
Proceeds from maturities of investments	75,165	31,821
Net cash used in investing activities	(29,805)	(21,365)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	125,417	50,327
Proceeds from the exercise of stock options	494	34
Net cash provided by financing activities	125,911	50,361
Net increase in cash and cash equivalents	59,990	10,931
Cash and cash equivalents at beginning of period	10,206	4,027
Cash and cash equivalents at end of period	70,196	$ 14,958
Non-cash financing activities
Conversion of preferred stock to common stock	$ 109,327