This subfamily includes Saccharomyces cerevisiae G2/mitotic-specific cyclins 1-4 (ScCLB1-4), S-phase entry cyclins 5-6 (ScCLB5-6), and Schizosaccharomyces pombe G2/mitotic-specific cyclins, cig1, cig2 and cdc13. ScCLB1-4 are essential for the control of the cell cycle at the G2/M (mitosis) transition. They interact with the CDC2 protein kinase to form maturation promoting factor (MPF). ScCLB5-6 interact with CDC28 and are involved in DNA replication in Saccharomyces cerevisiae. ScCLB5 is required for efficient progression through S phase and possibly for the normal progression through meiosis. ScCLB6 is involved in G1/S and or S phase progression. Cig1 is required for efficient passage of the G1/S transition. Cig2 and cdc13 are essential for the control of the cell cycle at the G2/M and G1/S (mitosis) transitions. They interact with the cdc2 protein kinase to form MPF. Members in this family contain two cyclin boxes. This model corresponds to the first one. The cyclin box is a protein binding domain.