sentences:  Cdc6 has been speculated to be a target of CDK action, because of the association between Cdc6 and CDK, and the CDK-dependent phosphorylation of Cdc6.The CDK-dependent phosphorylation of Cdc6 has been considered to be required for entry into the S phase.Both the catalytic subunits of DDK, Cdc7, and the activator protein, Dbf4, are conserved in eukaryotes and are required for the onset of S phase of the cell cycle.