CCNB3 (cyclin B3) is a cell cycle regulatory protein that functions as a positive regulatory subunit of cyclin-dependent kinase 1 (CDK1), controlling critical transitions during meiosis and mitosis 1. In oocyte maturation, CCNB3 regulates CDK1 activity, which modulates the anaphase-promoting complex/cyclosome (APC/C) through effects on CDH1 and CDC20 accumulation, ultimately controlling progression through meiosis I 1. Genetically, CCNB3 mutations cause female infertility through oocyte maturation arrest at the germinal vesicle or metaphase I stages due to aberrant CDK1 activity 1. In males, CCNB3 mutations contribute to globozoospermia, a form of teratozoospermia characterized by sperm morphological defects 2. Clinically, CCNB3 is notable as a fusion partner with BCOR in BCOR::CCNB3 sarcoma, a rare undifferentiated malignancy predominantly affecting males under age 20, primarily in bone and soft tissues 34. CCNB3 protein expression serves as a highly sensitive (93.3%) and specific (100%) diagnostic marker for BCOR::CCNB3 sarcoma by immunohistochemistry, showing diffuse nuclear staining 5. BCOR::CCNB3 sarcomas demonstrate approximately 75% five-year overall survival with Ewing sarcoma-type chemotherapy and wide resection, though local recurrence rates reach ~20% 4.