S期
In mid to late G1 phase, cyclin D bound to Cdk4/6, activates the expression of the S phase cyclin-Cdk components; however, the cell does not want S phase cyclins to become active in G1. Therefore, an inhibitor, protein Slc-1, is present that interacts with the dimer so that the S phase cyclin-Cdk dimer remains inactive until the cell is ready to move into S phase. After the cell has grown and is ready to synthesize DNA, G1 cyclin-Cdks phosphorylate the S phase cyclin inhibitor signaling ubiquitination, resulting in the addition of groups to the inhibitor. Ubiquitination of the inhibitor signals the SCF/proteasome to degrade the inhibitor releasing and allowing the S phase cyclin-Cdk to become activated and the cell moves into S phase. Once in S phase, cyclin-Cdks phosphorylate several factors on the replication complex promoting DNA replication by causing inhibitory proteins to fall off of replication complexes or through activation of components on the replication complex to induce DNA replication initiation.