Cyclin which controls both the G1/S and the G2/M transition phases of the cell cycle. Functions through the formation of specific serine/threonine kinase holoenzyme complexes with the cyclin-dependent protein kinases CDK1 and CDK2. The cyclin subunit confers the substrate specificity of these complexes and differentially interacts with and activates CDK1 and CDK2 throughout the cell cycle. Interacts with the CDK1 and CDK2 protein kinases to form serine/threonine kinase holoenzyme complexes. Ubiquitous. Accumulates steadily during G2 and is abruptly destroyed at mitosis. Belongs to the cyclin family. Cyclin AB subfamily.