Claudins are a family of tetraspan transmembrane proteins of the tight junctions. Tight junctions establish the paracellular barrier that controls the flow of molecules in the intercellular space between an epithelium’s cells. There are at least 24 Claudin proteins with organ-specific expression. Evidence of altered Claudin expression in various human malignancies has been accumulating.
Claudin 18 is one such protein, playing a key role in constructing tight junctions. There are two isoforms, Claudin 18.1 and Claudin 18.2. Claudin 18.1 is found primarily in the lungs. While in normal human tissues, expression of Claudin 18.2 is exclusively found in gastric epithelial cells and not in other organs, including the esophagus, colon, pancreas, and lung.