Essential for the control of the cell cycle at the G1/S (start) transition. Interacts with the CDC28 protein kinase to form MPF. CLN1 and CLN2 mRNAs fluctuate periodically in the cell cycle, peaking in G1 phase. A dominant mutation in CLN2 gene (CLN2-1), advances the G1- to S-phase transition in cycling cells and impairs the ability of cells to arrest in G1 phase in response to external signals. Present with 1270 molecules/cell in log phase SD medium. Belongs to the cyclin family.