Snail superfamily genes encode zinc finger-containing DNA binding proteins that act as transcriptional repressors. This superfamily has two main branches: the Snail family (encoded by the Snai1, 2 and 3 genes) and the Scratch family (the Scrt1 and 2 genes). These proteins are key regulators of the epithelial- mesenchymal transition, and also play roles in cell proliferation, survival and movement. We have performed a comprehensive genetic analysis of the requirements for Snail superfamily genes during embryonic development in mice. Our work during the prior funding periods of this grant established the null phenotype of all of these genes, and demonstrated that the most severe embryonic phenotype is observed in Snai1 mutants. However, many questions remain about the requirements and roles of Snail superfamily genes. In this proposal, both molecular and genetic approaches will be utilized to analyze the roles played by Snail family genes during embryogenesis in mice, and to understand the mechanisms causing the mutant phenotypes. The mutant mouse strains we have already generated, as well as the mutant strains we will construct as part of this proposal, constitute a unique set of reagents that will permit us to finely dissect the roles of Snail family genes during embryonic development. The specific aims of this proposal are: 1) determine targets for transcriptional repression by the SNAI1 protein during early embryogenesis in mice; 2) test the hypothesis that post- translational regulation of SNAI1 protein by the GSK3beta kinase and the betaTrcp ubiquitin ligase are essential for Snai1 function in vivo; 3) test the hypothesis that Snail family genes are important for muscle development, physiology and regeneration by determining the individual roles of the Snai1, Snai2 and Snai3 genes in these processes; 4) assess Snai3 redundancy with Snai2 and Snai1 function during muscle development, physiology and regeneration. Public Health Relevance: The long-term goal of this proposal is to understand the roles that Snail superfamily genes play during embryonic development in mammals. Snail superfamily genes encode zinc finger-containing DNA binding proteins that act as transcriptional repressors. The studies described in this proposal will further our understanding of the roles played by Snail superfamily genes during mammalian development, and will be relevant to the study of both normal and abnormal human development.