Interstitial lung diseases are characterized by chronic inflammation, fibrosis and damage to the lung parenchyma [78] . They consist of seven entities which are classified both by histology and clinicopathologic features [78] . In interstitial lung diseases, reports of claudin expression in UIP (Usual interstitial pneumonia) are present [18] . Changes in claudin expression mainly involve the regenerative alveolar cells or cells which have been replaced by metaplastic epithelia. In UIP regenerative metaplastic alveolar, squamous or bronchial epithelium shows strong reactivity for claudins 1, 2, 3, 4 and 7 compared to normal alveolar cells where only claudins 3, 4 and 7 are present [18] . There is a weaker reactivity for claudin 5 in such regenerative cells. In sarcoidosis, metaplastic regenerative epithelium displays a similar kind of reactivity in metaplastic cells as in UIP [18] . The formation of metaplastic epithelium in UIP probably is related to chronic inflammation and secretion of growth factors and integrins in the disease [78] . A different pattern of claudin expression in regenerative metaplastic cells compared to alveolar cells would lead to focal changes in the permeability of alveolar walls which is probably harmful for the function of the lung. There is also increased vascularity, neovascularisation and increased secretion of angiogenic factors in UIP [18] . Endothelial cells of blood vessels display intensive expression for claudin 5. According to some reports VEGF may upregulate claudin 5 [79] . Such increased staining may then be based on increased secretion of VEGF in UIP. Expression of claudins in different lung pathologies has been compiled in Table 2 .