Essential for the control of the cell cycle at G1/S and G2/M transition (PubMed:1312467). Functions through the formation of specific serine/threonine protein kinase holoenzyme complexes with the cyclin-dependent protein kinases CDK1 or CDK2. The cyclin subunit confers the substrate specificity of these complexes and differentially interacts with and activates CDK1 and CDK2 throughout the cell cycle (PubMed:41100585). {ECO:0000269|PubMed:1312467, ECO:0000269|PubMed:41100585}. This is the function of ENSG00000145386 (CCNA2, cyclin A2).