CLDN9 (claudin 9) is a tight junction protein that mediates cell-cell adhesion and forms paracellular barriers in epithelial tissues 1. While primarily functioning in maintaining tight junction integrity, CLDN9 has emerged as a significant oncogenic factor across multiple cancer types. In hepatocellular carcinoma (HCC), CLDN9 upregulation enhances metastatic potential through Tyk2/Stat3 signaling activation, promoting hepatocyte invasion and migration 2. Similarly, in glioblastoma multiforme, CLDN9 downregulation correlates with reduced tumor cell migration and invasion, and its expression inversely associates with activated CD8 T cells and dendritic cells, suggesting immunotherapeutic implications 3. In endometrial cancer, high CLDN9 expression independently predicts poor prognosis with a 5-year disease-specific survival rate of 62.8% versus 87.8% for low expression cases 4. CLDN9 dysregulation also appears prognostically significant in gastric cancer and non-functioning pituitary adenomas 567. Clinically, biallelic CLDN9 variants cause autosomal recessive deafness through impaired tight junction formation in inner ear epithelia, establishing CLDN9 as a bona fide deafness gene 1. CLDN9 shows promise as a multi-cancer biomarker and immunotherapeutic target for precision oncology approaches 8.