PCDHB9 encodes protocadherin beta 9, a transmembrane calcium-dependent cell adhesion protein 1. While UniProt annotations suggest involvement in neuronal synapse assembly, published evidence focuses exclusively on cancer biology. PCDHB9 regulates cell-cell adhesion through integrin signaling, as PCDHB9 knockdown significantly reduces adhesion to fibronectin and downregulates integrin subunit expression (ITGA3, ITGA4, ITGA5, ITGB1, ITGB6) 12. PCDHB9 is overexpressed in multiple cancers including colorectal 1, esophageal squamous cell 3, gastric 2, and prostate cancers 4, where it correlates with adverse clinicopathologic features and poor prognosis. Functionally, PCDHB9 promotes cancer cell migration, adhesion, and cancer stem cell properties (sphere formation) 32. In prostate cancer, high PCDHB9 expression associates with bicalutamide resistance and poor survival after androgen deprivation therapy 4. PCDHB9 expression also correlates with cisplatin resistance in gastric cancer 5. These findings suggest PCDHB9 functions as an oncogenic cell adhesion molecule that promotes metastatic progression and therapeutic resistance, positioning it as a potential therapeutic target across multiple cancer types.