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. When not in a complex with cdc2, interacts with spdya. Interacts with nap1l1. Interacts with nanos1. Accumulates steadily during G2 and is abruptly destroyed at mitosis. Belongs to the cyclin family. Cyclin AB subfamily.