Claudin 18 (CLDN18) belongs to the large claudin family of proteins, which are tetraspan transmembrane proteins of tight junctions (PMID: 11585919; 18036336). Claudins exhibit specific patterns of expression in different tissues (PMID: 15447685). CLDN18 is specifically expressed in the stomach and lung. CLDN18 has two alternatively spliced variants, CLDN18.1 and CLDN18.2. CLDN18.2 is a highly selective gastric lineage marker that determines the gastric phenotype in a neoplastic condition, whereas CLDN18.1 is lung specific (PMID: 11585919; 21832145). Altered CLDN18 expression has been reported in various human malignancies, including gastric cancer, lung adenocarcinoma, and pancreatic neoplasm (PMID: 17459057; 22076167; 21832145).