It is now clearly established that orderly progression through the eukaryotic cell cycle is due to regulation of cyclin dependent protein kinase (CDK) activity. Recently, it has become apparent that a number of mutations found in cancer cells are in, or directly affect the activity of; cell cycle control genes. The research described in this SBIR proposal will develop, for the first time, phenotypic assays for cyclin-CDK function in the yeast Saccharomyces cerevisiae. In Phase I, yeast strains in which reporter genes are expressed from promoters which respond to the activity of a yeast cyclin-CDK will be constructed. The LacZ reporter gene will provide a quantitative measure of cyclin-CDK function. The neo reporter gene will be used to assay lack of cyclin-CDK function, as well as provide a selection (G418 resistance) for loss of function mutants in either the cyclin or CDK. The cyclin binding domain of the yeast CDK will be mapped using the technology developed in Phase I. Because of the conservation of molecular mechanisms, including cell cycle control, between yeast cells and human cells, it is expected that the technology developed during Phase I can be adapted to the study of human cyclin-CDK function. PROPOSED COMMERCIAL APPLICATION: Development of phenotypic assays for cyclin-CDK function will facilitate cancer research and allow for the development, in Phase II, of high-throughput screening procedures for cancer therapeutics targeted to specific cyclin-CDK functions.