Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Collaboration revenue	$ 688	$ 567	$ 1,779	$ 567
Operating expenses:
Research and development	7,096	10,166	11,974	20,516
General and administrative	3,344	2,553	5,122	4,898
Total operating expenses	10,440	12,719	17,096	25,414
Loss from operations	(9,752)	(12,152)	(15,317)	(24,847)
Other income (expense):
Interest expense	(226)	(61)	(346)	(131)
Interest income	44	357	196	741
Loss before taxes	(9,934)	(11,856)	(15,467)	(24,237)
Income tax benefit	6	0	6	0
Net loss	(9,928)	(11,856)	(15,461)	(24,237)
Comprehensive income (loss):
Unrealized (loss) gain on investments	(1)	17	(16)	32
Unrealized gain (loss) on foreign currency translation	64	4	(49)	(10)
Comprehensive loss	$ (9,865)	$ (11,835)	$ (15,526)	$ (24,215)
Weighted-average shares used to compute basic and diluted net loss per share (in shares)	18,588,993	18,576,199	18,588,442	18,126,556
Basic and diluted net loss per share (in dollars per share)	$ (0.53)	$ (0.64)	$ (0.83)	$ (1.34)