The ocular lens provides an ideal system to study cell cycle regulation during terminal differentiation. The lens is composed of two cell types: the epithelial cells, which are capable of cellular proliferation, and the fiber cells, which are post-mitotic. I hypothesize that the differentiation of epithelial cells into fiber cells will cause alterations in activity of the genes that regulate the cell cycle. In order to help test this hypothesis, the alphaA-crystallin promoter will be used to direct lens-specific alterations in gene expression in transgenic mice. Transgenic and non-transgenic mice will be used to study two general classes of genes that are likely to be critical for cell cycle control during differentiation: tumor suppressors and cyclin-dependent kinases. The Specific Aims of this grant application are: l) to assess the role of the retinoblastoma (rb) protein in cell cycle control and terminal differentiation of lens fiber cells; 2) to assess the role of p53 in the induction of lens tumors by SV4O large T antigen; 3) to assay for changes in cyclin dependent kinases that accompany fiber cell differentiation; 4) to characterize the cell death that is induced by rb inactivation in fiber cells; and 5) to genetically reverse lens tumorigenesis induced by full-length T antigen. Our preliminary experiments have shown that expression of viral proteins that bind to rb and/or p53 can induce lens cell tumorigenesis or programmed cell death. Therefore, the cell cycle can be altered in lens cells with fascinating and unexpected consequences. The proposed studies should provide insights into cell cycle regulation in vivo, not only for the lens, but for other cells undergoing terminal differentiation or neoplastic transformation.