At the age of 16 weeks, each kitten of groups 1K, 10K, and 100K were infected once oronasally with FeLV-A/Glasgow-1 [19] by introducing 0.2 mL of virus suspension into each nostril and 0.6 mL into the mouth. The virus suspension for group 1K contained a total of 1 000 focus-forming units (FFU), the one for group 10K 10 000 FFU and group 100K 100 0000 FFU. The viral stock origin and infectivity was the same as described in earlier works [15, 41] .