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 the permeability between layers of myelin sheaths via focal adhesion. Its expression is highly regulated during development, suggesting that it may play an important role in the growth and differentiation of oligodendrocytes and other cells outside the CNS. In addition, the protein is a candidate autoantigen in the development of autoimmune demyelinating disease.