The transcriptional enhancer of C-type oncogenic retroviruses is an important genetic determinant of viral pathogenesis. Retroviral enhancers have been shown to influence leukemogenicity, the latent period of disease onset, and in some cases the cell type specificity of leukemia. Our long-term objective is to understand how the transcriptional enhancer confers these pathogenic phenotypes. Two protein binding sites in the Moloney murine leukemia virus (Moloney MLV) enhancer, the Leukemia virus factor b site (LVb) and the adjacent 'core' sequence, are major genetic determinants of the thymic disease specificity of the Moloney virus. The LVb site binds the Ets proteins. We and others have purified proteins that bind to the adjacent core site, and isolated cDNA clones encoding several of the core-binding factors (CBF). CBF consists of a DNA binding alpha subunit, and a beta subunit that associates with a CBF alpha subunit, but does not by itself bind DNA. Here we propose to continue characterizing the biochemical properties of the CBF alpha and beta subunits, and correlate these properties with viral pathogenesis. Specifically, we plan to: 1. Perform structure/function analyses of the CBF subunits. 2. Analyze the sequence specificity and binding affinity of various combinations of CBF alpha and beta subunits for the Moloney virus enhancer, and their contribution to pathogenesis. 3. Analyze the interaction between the Ets proteins and CBF. 4. Analyze the effects of a germline mutation in the CBF beta gene on T cell development and viral pathogenesis.