Coordination of cell division with growth and differentiation is critical to the development of multicellular organisms. The decision to enter or withdraw from the cell-cycle is made during G1 phase. The importance of this regulation is underscored by the finding that most tumor cells have defects in G1 control. In a screen for positive regulators of G1 progression, we identified mutations in
cyd-1 , the only C. elegans D-type cyclin, and
cdk-4 , the single kinase related to mammalian Cdk4 and Cdk6. Analysis of the mutant phenotype confirmed the previously published conclusion that
cyd-1 and
cdk-4 are required for G1 progression in postembryonic cell divisions (Park & Krause, Development 1999).   Members of the Rb tumor suppressor family are thought to be important targets of Cyclin D-dependent kinases. The synMuv class B gene
lin-35 is the single Rb-related gene in C. elegans (Lu & Horvitz, Cell 1998) . However, a role in cell-cycle regulation has not been described for
lin-35. Using double mutant combinations, we observed that
lin-35 inactivation overcomes the G1 arrest normally observed in
cyd-1 and
cdk-4 mutants. Thus,
lin-35 Rb is an important inhibitor of the G1/S transition and, by analogy with other systems, likely a major target of
cdk-4 and
cyd-1 . Interestingly, cell divisions are not restored to wild-type levels in
lin35;
cyd-1 or
lin-35;
cdk-4 double mutants, indicating that
cyd-1 and
cdk-4 have additional targets. We have identified a candidate additional target in a screen for suppressors of the
cyd-1;
lin-35 cell cycle defects.   As several synMuv genes encode homologs of known cell-cycle regulators, we tested synMuv class A and B genes for a role in regulating G1 progression. Loss of function of two class B genes,
lin-9 and
lin-36 , partially rescued the defects of
cyd-1 and
cdk-4 mutants. These results demonstrate a role for
lin-9 and
lin-36 as novel negative regulators of G1 progression.   As
lin-35 does not appear to provide the only level of control over G1 progression, we examined whether the Cdk inhibitors of Cip/Kip family cooperate with
lin-35 in controlling cell-cycle entry. The C. elegans genome encodes two Cip/Kip family members, CKI-1 and CKI-2. Combined inactivation of
lin-35 with
cki-1,2 dramatically increased the number of divisions of the intestinal nuclei compared to
lin-35 or
cki-1,2 mutants alone. Interestingly, a similar synergistic effect was observed between
lin-36 and
cki-1,2 . Our data shows that
cki-1,2 Cip/Kip cooperates with
lin-35 Rb and
lin-36 in controlling G1/S progression. Based on the overlap in phenotype of
lin-35 and
lin-36 mutants, it is likely that these two genes act in a linear pathway for cell-cycle control, perhaps even in a complex.