Essential for the control of the cell cycle at the G2/M and G1/S (mitosis) transition. Interacts with the cdc2 protein kinase to form MPF. Interaction with res2 promotes the phosphorylation of res1 and inhibits MBF-dependent gene transcription. Forms an autoregulating feedback-inhibition loop with MBF which is important for normal regulation of the cell cycle. G2/M cyclins accumulate steadily during G2 and are abruptly destroyed at mitosis. Negatively regulates conjugation via interacting with cell cycle 'start' genes. Degraded by skp1, pop1 and pop2 in the G2 and M phases of the cell cycle. Associates with cdc2, res2 and rum1. Interacts with pop1 only when phosphorylated. Highly induced upon nitrogen starvation and during conjugation. Phosphorylated. Belongs to the cyclin family. Cyclin AB subfamily.