Condensed Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Collaboration revenue	$ 1,746	$ 650	$ 4,645	$ 874
Revenue from Contract with Customer, Product and Service [Extensible List]	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember
Operating expenses:
Research and development	$ 9,455	$ 5,099	$ 23,983	$ 8,894
General and administrative	3,757	757	8,646	3,265
Total operating expenses	13,212	5,856	32,629	12,159
Loss from operations	(11,466)	(5,206)	(27,984)	(11,285)
Other income (expense):
Interest income	27	0	188	0
Interest and other expense	0	(1,462)	(209)	(3,049)
Change in fair value of preferred warrant liability	0	0	(75)	1
Total other income (expense)	27	(1,462)	(96)	(3,048)
Net loss	$ (11,439)	$ (6,668)	$ (28,080)	$ (14,333)
Net loss per share, basic and diluted	$ (0.31)	$ (2.43)	$ (1.72)	$ (5.32)
Weighted-average shares outstanding, basic and diluted	37,420	2,739	16,361	2,692