Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Collaboration revenue	$ 2,704	$ 1,358
Revenue from Contract with Customer, Product and Service [Extensible List]	rna:CollaborationRevenueMember	rna:CollaborationRevenueMember
Operating expenses:
Research and development	$ 20,677	$ 5,544
General and administrative	5,884	1,964
Total operating expenses	26,561	7,508
Loss from operations	(23,857)	(6,150)
Other income (expense):
Interest income	16	143
Interest expense	0	(78)
Other income (expense)	(3)	0
Total other income (expense)	13	65
Net loss	(23,844)	(6,085)
Other comprehensive income (loss):
Comprehensive loss	(23,842)	(6,085)
Net unrealized gains on marketable securities	$ 2	$ 0
Net loss per share, basic and diluted	$ (0.64)	$ (2.14)
Weighted-average shares outstanding, basic and diluted	37,521	2,845