In our study on metastatic adenocarcinomas and malignant mesotheliomas claudins 1, 3, 4, 5 and 7 had a lower expression in mesotheliomas suggesting that they could serve as differential diagnostic markers [107] . In mesothelioma subtypes, sarcomatoid and biphasic mesotheliomas showed less expression for claudins than epithelioid ones [107] . Non-neoplastic mesothelial cells showed expression for claudin 2 and weak expression for claudin 1, but no expression was found for claudins 3, 4, 5 or 7 [107] . In effusions claudins 3 and 7 significantly distinguished malignant Mesothelioma cells from metastatic adenocarcinoma cells [108] . Reactive mesothelial cells were negative for claudin 7 but showed infrequent claudin 1 and 3 expression. Thus also reactive mesothelial cells could be distinguished from metastatic cells in pleural effusions [108] . Metastatic ovarian carcinoma cells could also be distinguished from metastatic breast carcinoma cells by claudin 7, and generally, ovarian, endometrioid and cervical cancer metastases could be distinguished from lung or breast adenocarcinoma [108] . In the array study by Davidson et al serous mesotheliomas showed a significantly lower expression of claudins 3, 4 and 6 than ovarian adenocarcinomas speaking for a lower level of claudins in mesothelial derived neoplasms [108] . Facetti et al determined that especially claudin 4 could be used in differential diagnosis between malignant mesothelioma and metastatic adenocarcinoma [110] . Since adenocarcinoma metastases many times express claudin 4, CPE could potentially be used in treatment of such adenocarcinoma metastases in pleural cavity since benign mesothelial cells do not express this tight junctional protein [110] .