CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues:
Grant revenue	$ 2,089	$ 463	$ 6,000	$ 1,616
Collaboration revenue	$ 67		$ 3,874
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Total revenues	$ 2,156	$ 463	$ 9,874	$ 1,616
Operating expenses:
Research and development	12,026	7,374	21,552	16,299
General and administrative	3,782	3,060	7,670	6,104
Total operating expenses	15,808	10,434	29,222	22,403
Loss from operations	(13,652)	(9,971)	(19,348)	(20,787)
Other income (expense):
Interest income and other income (expense), net	502	15	1,126	187
Total other income (expense), net	502	15	1,126	187
Net loss	$ (13,150)	$ (9,956)	$ (18,222)	$ (20,600)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.74)	$ (0.69)	$ (1.04)	$ (1.43)
Weighted average common shares outstanding, basic and diluted:	17,667,620	14,376,529	17,445,600	14,372,876
Comprehensive loss:
Net loss	$ (13,150)	$ (9,956)	$ (18,222)	$ (20,600)
Other comprehensive gain (loss):
Unrealized gain (loss) on marketable securities	5	14	28	(15)
Reclassification adjustment for gains included in net loss			6
Net unrealized gains (losses) on securities	5	14	34	(15)
Total comprehensive loss	$ (13,145)	$ (9,942)	$ (18,188)	$ (20,615)