Furthering our present understanding of cell cycle by elucidating new regulatory mechanisms is critical for the subsequent development of novel therapeutic agents against cancer. The G1 phase of the cell cycle is an important intersection where positive and negative regulatory signals converge to control cell cycle progression. The negative regulation of the cell cycle is coordinated by the cyclin-dependent kinase (CDK) inhibitors and the family of pocket proteins. These proteins play a critical role during the G0/G1 phase of the cell cycle by negatively regulating the expression of E2F-responsive genes that are required for the G1/S transition. Presently, my work focuses on the negative regulation of the cell cycle by a mammalian form of C. elegans lin-9 and Drosophila mip130. In C. elegans, LIN-9 functions downstream of the mammalian equivalent of CDK4 in a pathway that regulates cell proliferation. Drosophila Mip130, along with Mip120 and Mip40, interacts with Drosophila Myb in a stable complex to regulate gene replication and transcription. The mammalian lin-9 homologue also appears to act downstream of cyclin D/CDK4 in mammalian cells. This is supported by the fact that over-expression of LIN-9 has an inhibitory effect on cell proliferation which is partially blocked by co-expression of cyclin D1 and by the partial rescue of phenotypic alterations in CDK-null mice by expression of a mutant form of LIN-9 lacking amino acids 1-84 (Æ84LIN-9). Moreover, we have determined that LIN-9 interacts with p107/p130, members of the pocket protein family and targets for CDK4 phosphorylation, and E2F4. Additionally, we have recently identified the stable interaction between LIN-9 and mammalian homologues of Drosophila Mip120 and Mip40. This complex of Drosophila mip homologues potentially acting in concert with p107/p130 and E2F4 to regulate mammalian gene transcription is unique and can further develop our present understanding of mammalian cell cycle regulation.