OIP5 (Opa interacting protein 5) is a multifunctional protein that plays critical roles in cell cycle regulation, centromere function, and cancer progression. The protein is required for proper chromosome 15 during mitosis and recruitment of CENPA to centromeres, as indicated by UniProt annotations. OIP5 functions as a key regulator of cell cycle progression by modulating cyclins A2, B1, D1, E1, and H, particularly cyclin D1 1. In glioblastoma, OIP5 forms a positive feedback loop with transcription factor E2F1, where E2F1 activates OIP5 expression, and OIP5 subsequently stabilizes E2F1 signaling to promote tumor progression and metastasis 2. The protein also interacts with NCK2 to regulate spermatogonial stem cell self-renewal and apoptosis 1. In bladder cancer, OIP5 contributes to chemoresistance by recruiting TRIP12 ubiquitin ligase to degrade phosphatase PPP1CB, enhancing YBX1 transcription factor activity and upregulating drug-resistance genes 3. Clinically, OIP5 is upregulated in multiple cancer types and correlates with poor prognosis 2. Mutations in OIP5 are associated with male infertility, specifically nonobstructive azoospermia, where reduced OIP5 expression is observed in affected patients 1.