Restricted expression of endogenous N‐Tropic XC‐positive leukemia virus in hybrids between G and AKR mice: An effect of the Fv‐4r gene

G mice carrying the Fv-4r gene are resistant to exogenous infections of various strains of ecotropic MuLVs (Suzuki, 1975). The expression of endogenous N-tropic XC-positive virus was studied in the progency of a cross between G and AKR mice. In the F1 mice, the virus expression was almost completely supressed, and no leukemia developed during 1 year of observation. Results of successive backcrossings with AKR mice indicated that a single dominant gene, the Fv-4r suffices for supression. It can be concluded that the extensive growth of N-tropic XC-positive virus is a prerequisite for leukemia development in AKR mice.