Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Collaboration revenue	$ 2,607	$ 1,541	$ 5,311	$ 2,899
Revenue from Contract with Customer, Product and Service [Extensible List]	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember
Operating expenses:
Research and development	$ 22,706	$ 8,984	$ 43,383	$ 14,528
General and administrative	6,268	2,925	12,152	4,889
Total operating expenses	28,974	11,909	55,535	19,417
Loss from operations	(26,367)	(10,368)	(50,224)	(16,518)
Other income (expense):
Interest income	16	18	32	161
Interest expense	0	(131)	0	(209)
Change in fair value of preferred warrant liability	0	(75)	0	(75)
Other expense	(2)	0	(5)	0
Total other income (expense)	14	(188)	27	(123)
Net loss	(26,353)	(10,556)	(50,197)	(16,641)
Other comprehensive income (loss):
Net unrealized gains on marketable securities	3	0	5	0
Comprehensive loss	$ (26,350)	$ (10,556)	$ (50,192)	$ (16,641)
Net loss per share, basic and diluted	$ (0.70)	$ (1.23)	$ (1.34)	$ (2.91)
Weighted-average shares outstanding, basic and diluted	37,583	8,586	37,552	5,715