The function of CCNB1 (cyclin B1, ENSG00000134057) is as follows. Essential for the control of the cell cycle at the G2/M (mitosis) transition (PubMed:27030811, PubMed:17495531, PubMed:17495533). Binds and activates cyclin-dependent protein kinase CDK1/CDC2 (PubMed:41100585). {ECO:0000269|PubMed:17495531, ECO:0000269|PubMed:17495533, ECO:0000269|PubMed:27030811, ECO:0000269|PubMed:41100585}.