The Claudin 18 IHC assay detects both isoforms of CLDN18: the CLDN18.1 isoform, which is predominantly expressed in normal and neoplastic lung tissue, and CLDN18.2, which is expressed in normal gastric tissue, gastric/GEJ adenocarcinoma, and other malignancies [20,21,22,23,24,25,26,27,28]. Here, CLDN18 is linked to gastroesophageal junction adenocarcinoma.