Background: Epidermal differentiation is a tightly regulated process that begins with actively dividing cells that eventually differentiates into enucleated corneocytes that form the water impermeable layer of the skin. Perturbations in the balance between growth and differentiation can lead to a variety of human skin disorders. Increased proliferation and diminished differentiation can lead to hyperproliferative disorders such as psoriasis, basal or squamous cell carcinomas. Tremendous effort has been placed on identifying tissue specific transcription factors that control epidermal differentiation. Much less emphasis has been placed on a potential role for epigenetic regulation of differentiation. In particular, the polycomb group (PCG) proteins offer an attractive means of repressing epidermal differentiation genes in the absence of differentiation inducing signals. Objective/Hypothesis: The overall goal of this proposal is to further define the factors involved in regulating epidermal differentiation, with a specific focus on the role of PCG proteins in epidermal differentiation. Our preliminary studies indicate that PCG proteins bind to a majority of epidermal differentiation genes in undifferentiated epithelial cells. We propose a model where PCG proteins actively repress epidermal differentiation genes in the undifferentiated state. Upon receiving differentiation signals, PCG proteins no longer associate with the promoters of differentiation genes allowing the chromatin to unwind to allow transcription of epidermal differentiation specific genes. Specific Aims and research design: (1) Genome wide mapping of PCG binding sites in the epidermis: 1A) To determine PCG binding regions in undifferentiated epidermal cells. 1B) To determine the impact of differentiation on PCG protein binding to previously identified (1A) target sites. (2) Characterization of the functional role of PCG proteins in epidermal differentiation: 2A) To determine the effect of loss of function of PCG proteins on epidermal differentiation. 2B) To determine the effect of gain of function of PCG proteins on epidermal differentiation. Relevance to public health: In almost all cancers, epigenetic changes accompany cancer initiation and progression. At the heart of these changes include altered epigenetic regulation mediated by polycomb group proteins. Components of the PCG protein complex such as EZH2 and Bmil are often overexpressed in cancer. Thus, understanding how PCG proteins regulate their target sites in normal cells is of utmost importance in determining how they are altered to contribute to cancer initiation and progression.