STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 37,975	$ 38,586	$ 46,305
General and administrative	10,879	10,636	10,219
Total operating expenses	48,854	49,222	56,524
Loss from operations	(48,854)	(49,222)	(56,524)
Interest income and other expense, net	2,182	2,283	861
Net loss	$ (46,672)	$ (46,939)	$ (55,663)
Net loss per share, basic and diluted (in dollars per share)	$ (1.59)	$ (1.71)	$ (2.72)
Shares used to compute net loss per share, basic and diluted (in shares)	29,349,810	27,509,960	20,488,506
Other comprehensive loss:
Unrealized gain (loss) on marketable securities	$ 63	$ 7	$ (2)
Comprehensive loss	$ (46,609)	$ (46,932)	$ (55,665)