PCDH9 (protocadherin 9) is a calcium-dependent cell adhesion molecule belonging to the δ1-protocadherin family that plays critical roles in neural development and tumor suppression 1. The protein functions through dual mechanisms of extracellular calcium-dependent adhesion and intracellular signal transduction 2. In the nervous system, PCDH9 is predominantly expressed in brain regions including hippocampus and prefrontal cortex, where it regulates cell migration, circuit formation, and maintenance 13. PCDH9 deficiency is associated with neurological disorders including major depressive disorder, autism-spectrum behaviors, and disrupted hippocampal migration 42. In cancer biology, PCDH9 acts as a tumor suppressor by inhibiting epithelial-mesenchymal transition, cell cycle progression, and invasion in gliomas and other cancers via the GSK-3β/Snail1 axis 25. PCDH9 expression is tightly regulated by microRNAs such as miR-215-5p and epigenetic mechanisms, with downregulation contributing to glioma progression 6. The protein also undergoes ubiquitination and degradation by COP1 E3 ligase 7. Additionally, PCDH9 expression patterns during kidney development suggest involvement in congenital anomalies of the kidney and urinary tract 8.