TABLE 1.

Occurrence of a second-site compensatory C-U₂₂₇₈ mutation in editing-site mutants at various passages

Mutant virus	Passagea	Total no. of coloniesb	No. of colonies positive for:			% C-U mutation
			P	V	W
rNDV	3	116	79	32	5	0
NDV-P1	5	56	56	0	0	0
NDV-P1	6	102	101	1	0	4
NDV-P1	7	96	90	5	1	88
NDV-PC4	3	29	29	0	0	0
NDV-PC4	7	28	27	1	0	89
NDV-T2	2	27	4	12	11	100c

^aNumbers of serial passages in 9- to 11-day-old embryonated SPF chicken eggs.

^bTotal number of independent colonies sequenced to determine the occurrence of a mutation.

^cThe C-U₂₂₇₈ mutation was artificially introduced.