Essential for the control of the cell cycle at the G2/M (mitosis) transition. Interacts with the CDC2 protein kinase to form a serine/threonine kinase holoenzyme complex also known as maturation promoting factor (MPF). The cyclin subunit imparts substrate specificity to the complex. Accumulates steadily during G2 and is abruptly destroyed at mitosis. Belongs to the cyclin family. Cyclin AB subfamily.