The claudin superfamily consists of many structurally related proteins in humans. These proteins, which include claudin 1 through 18, 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. Claudins, which consist of four trans- membrane domains and two extracellular loops, make up tight junction strands. Claudin-11 is an oligodendrocyte-specific protein that is expressed in the tight junctions of Sertoli cells and myelin sheaths in mice.