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cyclin Any of a family of proteins that help control the various phases of the cell cycle. Their concentrations fluctuate in step with the cycle, providing the cues for a progression from mitosis to the G1, S, and G2 phases during each complete cycle. They act in conjunction with cyclin-dependent protein kinases, which are proteins that phosphorylate other proteins. For example, in all eukaryotes mitosis (M phase) is initiated by high levels of cyclin B, which combines with a protein kinase to form the mitosis-promoting factor (MPF). By the end of the M phase, cyclin B is at a very low concentration; thereafter it rises steadily again to peak just before M phase.