The mechanisms that control cell-cycle entry during the G1 phase are fundamental to normal development and frequently lost in tumor cells. A substantial number of genes have been identified that affect G1 progression of cells in tissue culture. However, how such genes act in vivo and how they form part of the regulatory networks that control cell-cycle entry during development remains poorly understood. Genetic model systems have provided powerful means to address these questions. In this grant proposal, we describe genetic approaches in the nematode C. elegans aimed at identifying genes with critical G1 control functions in vivo and characterizing how these genes act together with known components in the regulation of G1 progression. We have already characterized several genes in C. elegans with critical roles in G1 progression. These include positive regulators homologous to a D-type cyclin and a Cdk4/6 kinase, and negative regulators related to the retinoblastoma (Rb) protein and the p21/p27 CIP/KIP family of CDK inhibitors. Our results indicate that G1 control in C. elegans follows molecular mechanisms closely related to those used in mammalian cells. Thus, results obtained in C. elegans are likely to apply to G1 regulation of normal and cancer cells in humans. We have established that the Rb and Cip/Kip family members contribute non-overlapping levels of G1 inhibition. In addition, we have identified roles for the novel genes lin-9 and lin-36 as negative regulators of the G1/S transition. In this proposal we outline experiments to identify and characterize critical components of the Cip/Kip pathway, the Rb pathway and additional pathways for G1 control in C. elegans. Specific Aim 1 will focus on the molecular and genetic characterizations of the cell-cycle functions of lin-9, lin-35 Rb and lin-36. Specific Aim 2 describes genetic screens aimed at identifying genes that act downstream of or in parallel to the cyd-1/cdk-4 kinase. Pilot screens have already produced highly interesting candidate mutations. Mutations will be characterized, mapped and assigned to complementation groups. As described in Aim 3, we will place novel cell-cycle regulators into pathways, initiate molecular characterizations and examine their role in human cancer.