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. Claudins, which consist of four transmembrane domains and two extracellular loops make up tight junction strands. Claudin expression is often highly restricted to specfic regions of different tissues and may have an important role in transcellular transport through tight junctions. Claudin-6 is expressed in differentiated F9 cells that resemble tight junction-bearing viceral endoderm resulting from stimulation with retinoc acid and mediated by RXRalpha and RARgamma . Claudin-6 is absent in mouse brain and lung. The human claudin-6 gene maps to chromosome 16p13.3.