CLDN17 (claudin-17) is a tight junction protein that functions as an anion-selective paracellular channel, preferentially permitting chloride and bicarbonate ions smaller than 9 Ångströms in diameter while excluding water 1. In the kidney proximal tubule, CLDN17 likely mediates paracellular reabsorption of filtered anions through a hydrophilic furrow mechanism involving key residues K65, R31, E48, and Y149 1. CLDN17 demonstrates conflicting roles in cancer biology. In hepatocellular carcinoma (HCC), CLDN17 is upregulated and promotes malignancy by activating the Tyk2/Stat3 signaling pathway, enhancing hepatocyte migration and predicting poor prognosis 23. Conversely, in oral cancer, CLDN17 acts as a tumor suppressor—its downregulation correlates with advanced disease, and CLDN17 overexpression inhibits epithelial-mesenchymal transition (EMT), invasion, and migration 4. Clinically, CLDN17 autoantibodies show diagnostic promise in pancreatic adenocarcinoma. A multivariate signature combining CLDN17 autoantibodies with CA19-9 achieved superior diagnostic accuracy (AUC = 0.97) compared to individual markers alone 56. Additionally, CLDN17 variants associate with stroke risk in African American populations, suggesting roles in blood-brain barrier permeability 7.