Claudins are a family of proteins that are the most important components of the tight junctions, where they establish the paracellular barrier that controls the flow of molecules in the intercellular space between the cells of an epithelium. 23 claudins have been identified. They are small (20-27 kilodalton (kDa)) proteins with very similar structure. They have four transmembrane domains, with the N-terminus and the C-terminus in the cytoplasm. Claudin-1 is expressed primarily in keratinocytes and normal mammary epithelial cells.