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The Cdc2–Cdc13 complex formed is not necessarily active, because cells have at least three different mechanisms to down-regulate its activity. 1/ Degradation of Cdc13 subunit, which is mediated by the APC (anaphase-promoting complex), which requires Slp1 protein to recognize Cdc13. Degradation of Cdc13 is primarily responsible for the Finish transition, 2/ Inhibitory phosphorylation, which is mediated by Wee1 kinase, whose action is reversed by Cdc25 activatory phosphatase. Inhibitory phosphorylation is used for the regulation of the G2/M transition. 3/ Binding of an inhibitor (Rum1) to the Cdc2–Cdc13 complex. Rum1 is continuously synthesized; when it becomes phosphorylated, it is rapidly destroyed. The Rum1 Cdk-inhibitor stabilizes the G1 state and is destroyed at the Start transition. G1 is also stabilized by cyclin proteolysis, mediated by Ste9/APC (like Slp1, Ste9 is used by APC to recognize Cdc13).