Essential for the control of the cell cycle at the G2/M (mitosis) transition. Interacts with the cdk1 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. Very low levels are seen in stationary-phase cells (0 hour) which then increase to a maximum between 5 and 6 hours (at which time cells begin to divide). Protein levels then decrease until the end of cell division. Belongs to the cyclin family. Cyclin AB subfamily.