STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities
Net loss	$ (34,347)	$ (24,906)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	167	112
Stock-based compensation expense	3,672	3,099
Loss on disposal of fixed assets		42
Noncash lease expense	127	107
Noncash lease liability	187	(143)
Amortization of discount on investments	2
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(680)	(1,095)
Accounts payable, accrued expenses and other long term-liabilities	827	2,404
Net cash used in operating activities	(30,045)	(20,380)
Cash flows from investing activities
Maturities of short-term investments	2,729	5,000
Purchases of short-term investments	(8,931)	(2,482)
Deposit on property and equipment		(6)
Purchases of property and equipment		(340)
Net cash (used in) provided by investing activities	(6,202)	2,172
Cash flows from financing activities
Proceeds from exercise of stock options	512	65
Proceeds from equity offerings, net of offering costs	43,722	(148)
Net cash provided by (used in) financing activities	44,234	(83)
Net increase (decrease) in cash and cash equivalents	7,987	(18,291)
Cash and cash equivalents-beginning of period	90,943	67,727
Cash and cash equivalents-end of period	98,930	49,436
Supplemental disclosure of cash flow information
Conversion of preferred stock to common stock	17
Accrued equity offering costs	$ 210
Operating lease liability		3,357
Operating right-of-use asset		$ 2,458