During the last two years these investigators have learned several key properties of the cyclin-dependent kinases that have helped understand how these kinases recognize their substrates and how they might develop technical approaches to allow rapid identification of substrates for the cell-cycle-regulating kinases. The investigators have developed two techniques for screening of new substrates of the CDK2 kinases, enzymes that control the last steps of G1 and the early steps of DNA synthesis. They have identified six new potential cyclin E/CDK2 substrates to date. Two of the new substrates have been characterized in sufficient detail to believe that they are in vivo substrates. In this application, it is proposed to continue examination of how cyclin/CDK complexes recognize their substrates, to expand the validation of the substrate screening methods, to extend these screens to look for substrates for a limited set of other CDK family members, and to study the function of the new substrates through these screens.