Oligodendrocyte specific protein (OSP)is the third most abundant component of central nervous system myelin. The claudin superfamily consists of many structurally related proteins. These proteins, which include claudin 1 through 18, are located in both epithelial and endothelial cells in all tight junction bearing tissues. Claudins, which consist of four transmembrane domains and two extracellular loops, make up tight junction strands. There is growing evidence that OSP/Claudin 11 determines