Claudins are the major cell adhesion molecules in tight junctions and are involved in regulating the paracellular flux of water-soluble molecules between adjacent cells (reviewed in (Furuse and Tsukita, 2006). Claudins create paired strands through homophilic and heterophilic cis and trans interactions. A strand of one cell associates laterally with a strand in the apposing membrane of an adjacent cell creating a paired TJ strand (Tsukita et al., 2001). The TJ strands contain aqueous pores with size and charge selectivity that are permeable to water-soluble molecules. Differences in the barrier properties in epithelia of different tissues have been explained by the expression of a unique set of claudins in a given tissue (Van Itallie and Anderson, 2006). 24 claudins were identified in humans, which allows a large number of possible combinations and specific barrier properties.