Condensed Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ 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	$ 1,541	$ 224	$ 2,899	$ 224
Revenue from Contract with Customer, Product and Service [Extensible List]	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember
Operating expenses:
Research and development	$ 8,984	$ 2,533	$ 14,528	$ 3,795
General and administrative	2,925	1,596	4,889	2,508
Total operating expenses	11,909	4,129	19,417	6,303
Loss from operations	(10,368)	(3,905)	(16,518)	(6,079)
Other income (expense):
Interest income	18	0	161	0
Interest and other expense	(131)	(1,215)	(209)	(1,587)
Change in fair value of preferred warrant liability	(75)	4	(75)	1
Total other income (expense)	(188)	(1,211)	(123)	(1,586)
Net loss	$ (10,556)	$ (5,116)	$ (16,641)	$ (7,665)
Net loss per share, basic and diluted	$ (1.23)	$ (1.89)	$ (2.91)	$ (2.87)
Weighted-average shares outstanding, basic and diluted	8,586	2,704	5,715	2,669