Claudins are involved in embryogenesis and organogenesis and changes in their expression can be seen in the organogenesis of the gut and kidney [23, 28] . Claudin 15 serves as a target for tcf2 gene in zebrafish creating one lumen instead of multiple ones [29] . Interestingly, claudin 15 knockout mice develop a megaintestine [30] . In organogenesis, claudin expression is turned off in cells taking part in epitheliomesenchymal transition [31] . In embryonic development claudins regulate cellular integrity, and create hydrostatic pressure differentials maintaining integrity of cystic compartments [31, 32] . Additionally, claudin 1 may have a role in right-left patterning of the embryonic tissues [31] . In Xenopus, claudin 5 is required for heart development [33] .