We propose to analyze cellular factors in the mouse which control replication of exogenous murine leukemia virus (MuLV) and the induction of endogenous MuLV. We plan to identify the chromosomal locations of endogenous ecotropic and xenotropic viruses by means of analysis of mouse-Chinese hamster hybrids which segregate murine chromosomes. The gene which governs the receptor for ecotropic MuLV has already been mapped to chromosome 5 using this technique. Material for this study will be used to immunize Chinese hamsters to prepare an antireceptor antibody. Spleen cells from immunized hamsters will be hybridized in vitro with amyeloma to produce large quantities of homogenous antibody. Analysis of expression and isolation of the receptor will be feasible with this reagent. Studies will be carried out to identify the chromosomal location of endogenous virus by analysis with monoclonal antibodies directed against individual viral proteins. Viral sequences will be mapped by DNA-DNA hybridization of hybrid cell nucleic acid and by restriction endonuclease digestion.