Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Collaboration and contract research revenue	$ 6,787	$ 2,319	$ 379
Revenue from Contract with Customer, Product and Service [Extensible List]	rna:CollaborationAndContractResearchRevenueMember	rna:CollaborationAndContractResearchRevenueMember	rna:CollaborationAndContractResearchRevenueMember
Operating expenses:
Research and development	$ 37,602	$ 14,539	$ 8,436
General and administrative	13,462	5,112	2,441
Total operating expenses	51,064	19,651	10,877
Loss from operations	(44,277)	(17,332)	(10,498)
Other income (expense):
Interest income	206
Interest and other expense	(209)	(7,387)	(718)
Change in fair value of preferred warrant liability	(75)	(15)	0
Total other income (expense)	(78)	(7,402)	(718)
Net loss	(44,355)	(24,734)	(11,216)
Other comprehensive income (loss):
Net unrealized losses on marketable securities	(5)
Comprehensive loss	$ (44,360)	$ (24,734)	$ (11,216)
Net loss per share/unit, basic and diluted	$ (2.05)	$ (9.12)	$ (4.53)
Weighted-average shares/units outstanding, basic and diluted	21,663	2,713	2,477