The role of p27Kip1 in preventing premature activation of S-phase. Cdk1:CcnA and Cdk1:CcnB activities are both required to induce mitosis during mammalian cell proliferation. Either activity is required to activate the APCCdc20 ubiquitin ligase which, in turn, targets the mitotic cyclins A and B for ubiquitin mediated degradation by the 26S proteasome. Cdk1:CcnA also phosphorylates the Orc1 subunit and thereby prevents ORC binding to chromatin until metaphase is completed and APCCdc20 inhibits Cdk1 activity by reducing the pools of mitotic cyclins A and B. Once this occurs, and the spindle checkpoint releases APCCdc20, cells move into anaphase and APCCdc20 targets geminin for degradation. In the absence of CDK activity, Orc1 and Cdc6 are dephosphorylated and bind to chromatin. In the absence of geminin, Cdt1 loads the MCM DNA helicase onto ORC•Cdc6•chromatin sites. Cdk1 protein is inactivated by the Wee1 and Myt1 protein kinases. APCCdh1 appears and targets Cdc20 for ubiquitin-dependent degradation. The cells are now maintained in G1-phase by APCCdh1 preventing the synthesis of cyclins A and B. However, cyclin E is not an APC substrate. As cyclin E appears, Cdk2:CcnE activity activates the replication complexes and drives the cells into S-phase. The role of p27 is to prevent premature accumulation of Cdk2:CcnE activity.