Cell cycle control in higher eukaryotes. (Top) Cyclin-CDK complexes in higher eukaryotes, and their approximate times of activity during the cell cycle. For clarity, extended cyclin families are indicated only by their class name (i.e. cyclin D rather than cyclin D1, D2, D3). The two known families of CDK inhibitors are also indicated. R indicates the Restriction point, beyond which cells do not required growth-factor signaling to complete cell division. (Bottom) Control of Cdk1 activity by phosphorylation. Phosphorylation of a conserved Thr residue by CAK is required for full activation. Phosphorylation of Tyr15 by Wee1 or both Thr14 and Tyr15 by Myt1 blocks Cdk1 activity, and is counteracted by members of the Cdc25 family of phosphatases.