In immunohistochemical studies on human alveolar cells, claudins 3, 4 and 7 have been detected in type 2 alveolar cells while type 1 alveolar cells were negative [17, 18] . Rat alveolar epithelial cells expressed claudin 3, 4 and 7 mRNA but claudin 1 or claudin 5 mRNA were not found even though claudin 5 was found in western blot analysis in alveolar cells [25] . Rat type 2 alveolar cells express more claudin 3 and type 1 cells more claudin 7 [16, 26] . Thus there are inconsistencies regarding claudin expression also in alveolar cells detected by immunohistochemistry or mRNA analysis which may be species specific, partly due to the method used or may reflect extended half lives of proteins compared to RNA. Inconsistent results between protein and mRNA levels of claudins 1, 3 and 7 have also been detected in alcohol induced lung changes in rats where the protein expression of these claudins increased while the mRNA levels stayed the same [27] . In studies on claudin mRNA expression in human lung tissues claudins 6, 9, 10, 11, 15 or 16 have not been detected [14, 16] . Expression of claudins in lung cells has been compiled in Table 1 .