CCNB1 (Cyclin B1) is a regulatory protein that functions in the mitotic switch in cell division, and also participates in nuclear envelope breakdown, spindle pole assembly and chromosome condensation. CCNB1 complexes with CDK1 (p34) to form the maturation-promoting factor (MPF). CCNB1 is upregulated in numerous cancers, including breast, gastric, colorectal, lung, prostate, and cervical carcinomas, where high levels of CCNB1 cause uncontrolled tumor cell growth alongside inactivation of TP53. In immunohistochemistry, CCNB1 has cytoplasmic positivity in proliferating cells in most tissues throughout the body. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804;