cyclin

Proteins (A and B forms known) whose levels in a cell varies markedly during the cell cycle, rising steadily until mitosis, then falling abruptly to zero. As cyclins reach a threshold level, they are thought to drive cells into G2 phase and thus towards mitosis. Cyclins combine with p34 kinase (cdc2) to form maturation promoting factor (MPF). See also M-phase promoting factor, cyclin-dependent kinase (cdk), cdk inhibitors, cdk activating kinase.