A family of cell-cycle regulatory proteins that are required by CDK (cyclin-dependent kinase) proteins for their activity. Entry into and transit through the cell cycle is a highly regulated process, to a large degree contolled by the phosphorylating activity of cyclin-cdk complexes. The cyclins themselves are regulated in a cell-cycle dependent manner. The cyclin-cdk complexes activate proteolytic pathways that lead to cyclin degradation, thus forming an autoregulatory feedback loop.

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