Tight junctions (TJs) play a pivotal role in compartmentalization in multicellular organisms by sealing the paracellular pathway in epithelial and endothelial cell sheets. Recently, novel integral membrane proteins, claudins, have been identified as major cell adhesion molecules working at TJs. Claudins comprise a multigene family, and each member of approximately 23 kDa bears four transmembrane domains. To date, 15 members of this gene family have been identified. When expression vectors of each species of claudins were transfected into fibroblasts lacking endogenous claudins or TJs, well-developed TJs were observed between adjacent transfectants. Furthermore, claudins were shown to be directly involved in the barrier function of TJs by experiments using Clostridium perfringens enterotoxin. Now that claudins have been identified, the structure and functions of TJs should be determined in detail in molecular terms.