The objective of this work is to identify the gene or genes responsible for the target cell specificity and transforming potential of hematopoietic cells by murine leukemia viruses (MuLV's). We will concentrate on the study of mink cell focus-inducing viruses (MCF's) and the defective spleen focus-forming viruses (SFFV's). We will study the target cell specificity for transformation by using: 1) pseudotypes containing the gemone of a virus with one target cell specificity and the coat from a virus with another target cell specificity; 2) recombinant viruses formed in tissue culture between a virus of one tropism and a virus of another propism; 3) recombinant viruses obtained by ligation of portions of cloned viral DNA's derived from viruses of different target cell specificity; 4) recombinant viruses obtained by marker rescue. We will study the transforming ability and the target cell specificity of the four different components of an in vivo passaged Rauscher-MuLV complex by in vivo and in vitro assay.