purpose of this project is to study the effect of constitutive cyclin E expression on mitotic division and to ultimately identify the mechanism through which cyclin E leads to chromosome instability. Cyclin E functions in timing the GI/S phase transition and centrosome duplication, but when cyclin E expression is deregulated cells enter S phase and exit more slowly and exhibit a moderate level of chromosome Instability. We propose that cyclin E may be interfering with mitotic division leading

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... chromosome instability and eventual tumorigenesis. In the second year of funding, I have accomplished my goals including, finishing documentation of mitotic delays using immunofluorescence and live cell microscopy, quantification of centrosome aberrations, and using biochemical assays to analyze expression of mitotic regulators in order to help in the identification of possible substrates of cyclin E/Cdk2 phosphorylation in mitosis. Finally, important progress has been made toward understanding how cyclin E delays mitosis. Using siRNA against BubR1 to disrupt the spindle checkpoint, the effects previously observed with deregulated cyclin E expression were reversed, showing that cyclin E is working through the spindle checkpoint to delay mitotic cells.