The Claudin superfamily consists of many structurally related proteins in humans. These proteins are important structural and functional components of tight junctions in paracellular transport. Claudins are located in both epithelial and endothelial cells in all tight junction-bearing tissues. Three classes of proteins are known to localize to tight junctions, including the Claudins, Occludin and junction adhesion molecule (JAM). Claudins, which consist of four transmembrane domains and two extracellular loops, make up tight junction strands. Claudin expression is highly restricted to specfic regions of different tissues and variations of Claudin expression may have an important role in transcellular transport through tight junctions. In rat liver, claudin-3 is uniformly expressed, whereas in the pancreas, claudin-3 is expressed in junctions of the duct epithelia and junctions of acinar cells. Claudin-3 binds the peptide toxin Clostridium perfringens enterotoxin (CPE) at the cell surface via the second extracellular loop of claudin-3. The gene encoding human claudin-3 maps to chromosome 7q11.23.