Lung carcinoma cells contain tight junctions and their number is inversely associated with patient prognosis and aggressivity [80] . In lung tumor cell lines, variable expression of claudins 1, 2, 3, 4 and 7 is found [40] . In studies on lung tumor material different histological tumor types vary in their claudin expression showing up-or downregulation of different claudins compared to normal bronchial cells or lung tissue [40, [81] [82] [83] . In the study of Moldway et al small cell lung carcinomas show a 16 fold higher level of claudin 3 mRNA expression than normal lung tissue [82] . Claudin 4 mRNA is upregulated to 3-4 fold level in squamous, adenocarcinoma and small cell carcinoma compared to normal lung [82] . Both adeno-and squamous cell carcinoma showed a slight downregulation of claudin 1 mRNA compared to normal lung and squamous cell carcinomas had a 2.7 fold level of claudin 1 mRNA than adenocarcinomas [82] . Paschoud et al found an increase in claudin 5 and a decrease in claudins 3, 4 and 7 mRNA in squamous cell carcinoma compared to normal bronchial cells and an increase in claudin 1 and a decrease in claudin 5 compared to lung parenchyma [81] . Compared to bronchial cells claudins 1, 3 and 4 and 7 were decreased and claudin 5 mRNA increased in adenocarcinoma while when compared to lung parenchyma claudin 4 mRNA was increased but claudin 5 decreased [81] .