. "The murine leukemia virus, E-55+ virus, induces a thymic lymphoma/leukemia in 100% of BALB.K mice infected as adults after a latent period of 4 months or more (Pozsgay et al., Virology 173, 330-334, 1989). Two molecular clones of virus designated E-55+ and E-55-based on their ability to encode the E-55 epitope detected by the monoclonal antibody 55 (mAb 55) were isolated from a leukemic BALB.K mouse inoculated with a biologically cloned E-55+ virus (Chesebro et al., Virology 112, 131-144, 1981). Env gene sequence analysis of E-55+ and E-55-clones showed that the E-55-virus was generated from the E-55+ virus as the result of a recombination between E-55+ virus and the endogenous ecotropic virus, emv-1, carried in the genome of the BALB.K mouse strain. The recombinant E-55-virus is replication competent. This recombination event and the consequential expression of E-55-virus consistently occur in immunocompetent BALB.K mice inoculated with the E-55+ virus and appear to play a role in the loss of epitopes recognized by virus neutralizing antibodies. The loss of these epitopes apparently allows the virus to evade the host immune response." . . . "Virology" . "Prystowsky M.B." . "Blank K.J." . "Tumas K.M." . "Poszgay J.M." . "Avidan N." . "Ksiazek S.J." . "Overmoyer B." . "1993"^^ . "587-595" . "Loss of antigenic epitopes as the result of env gene recombination in retrovirus-induced leukemia in immunocompetent mice." . "192" . "doi:10.1006/viro.1993.1075" .