Cancer results from genetic and epigenetic changes that cause uncontrolled proliferation of cells. Central to this process are changes in the expression of tumor suppressor genes or oncogenes. P53 is one of the best-characterized tumor suppressor genes, whose expression is lost or mutated in approximately 50% of human cancers. P63 is a member of the p53 gene family that plays critical roles in epithelial cell proliferation and homeostasis, through regulation of cellular senescence and stem cell renewal. However the role of p63 during tumor development is unclear, with both tumor suppressive and oncogenic roles attributed. The p63 protein is overexpressed in many tumors, particularly squamous cell carcinoma of the skin and lungs. However, p63 expression is lost during the development of adenocarcinoma of the prostate and breast. The work outlined in this proposal aims to determine the role of p63 in the prostate gland, particularly in prostate stem cells, and during development of prostate tumors, using a combination of cell and molecular biology, 3D tissue culture and prostate-specific mouse models. The expression pattern of the six different isoforms of p63 will be determined in the normal prostate gland and in prostate stem cells. Loss-of-function studies will be performed in vivo to address the role of p63 in the prostate gland. Using prostate-specific mouse models, p63 will be ablated in the normal prostate gland and during prostate tumor development. The effects of loss of p63 on prostate stem cell proliferation, prostate tumor initiation, maintenance and metastasis will be examined. High-throughput genomic screens will also be performed to identify novel p63-target genes in the prostate gland and to determine the functions of these genes in stem cell proliferation and during tumor development. It is anticipated that this work will identify key genes and mechanisms during prostate tumor development that can be targeted for therapy in future studies.