CLDN18 and esophageal adenocarcinoma: CLDN18.2 is the dominant claudin 18 (CLDN18) isoform expressed in both normal and malignant gastric epithelial cells and is also frequently expressed in tumors derived from organs that normally do not express CLDN18.2, including pancreatic and esophageal adenocarcinomas [12, 20–23].