PCDHA9 (protocadherin alpha 9) is a calcium-dependent cell adhesion protein involved in establishing and maintaining neuronal connections 1. It localizes to the plasma membrane and mediates cell adhesion through calcium-dependent interactions [GO annotations cited in abstracts]. Mechanistically, PCDHA9 functions in cell adhesion and synapse organization. In the enteric nervous system, PCDHA9 is expressed in the myenteric plexus and regulates enteric neuron proliferation and apoptosis 2. At the neuromuscular junction, PCDHA9 mutations impair NKA-α1 expression and disrupt ion transport and neuronal survival 1. PCDHA9 dysfunction is implicated in multiple diseases. Homozygous mutations cause progressive motor neuron degeneration with TDP-43 pathology and paralysis in mice 1. Heterozygous mutations contribute to hypoplastic left heart syndrome and bicuspid aortic valve through disrupted aortic development 3. PCDHA9 mutations are also associated with Hirschsprung's disease 2, short root anomaly in tooth development 4, and reduced male fertility through microRNA-23a/b-3p regulation 5. Additionally, PCDHA9 downregulation correlates with poor prostate cancer prognosis 6. Clinically, PCDHA9 variants represent a genetic risk factor across neurodevelopmental, cardiac, and reproductive disorders, suggesting therapeutic potential through restoration of adhesion-mediated signaling.