The Fv1 specific inhibition of MLV infection. There are two major alleles of the gene Fv1 among inbred mice, Fv1n and Fv1b. Mice that carry the Fv1n allele are protected from B-tropic MLV infection (MLV B), whereas those carrying the Fv1n allele cannot be infected by N-tropic strains (MLV N). See text for detailed discussion. Abbreviations: MLV, murine leukemia virus.