Transformation by retroviruses without oncogenes has been extensively studied. Mink cell focus-inducing (MCF's) viruses are members of this group of viruses and are consistently isolated from T cell lymphomas of certain strains of inbred mice (AKR's). My previous work demonstrates that at least four regions (gp70, p15E, LTR and gag) of MCF 247, an oncogenic MCF virus, control the oncogenic potential of the virus. The function of these genes has been elusive in part because it has been difficult to track the fate of MCF viruses in vivo. WE have now developed a method to tag MCF viruses with a suppresor tRNA gene (supF) and to track and recover the viruses after they are injected into AKR mice. Preliminary evidence indicates that recombination of the env gene and duplication of the enhancer sequences occur. We propose now: (1) to determine if these recombination and duplication events are required events during the preleukemic period and whether the changes in the viral genome are correlated with common integration sites near specific protooncogenes, (2) to characterize viral infection and transcription in specific thymocyte subpopulations, as a first step in understanding the early cellular virus- induced changes in thymocytes. (3) to isolate and characterize the gene for the MCF receptor. We expect that the results of these experiments will lead to a more complete understanding of the complex in vivo interactions of viral genes with specific cell types and will ultimately allow us to understand the relationship of the function of viral genes to transformation.