Claudin-5 was mainly positive in the endothelial cells of adult human lung, sarcoidosis and IPF, but it was also faintly positive in the bronchial and metaplastic alveolar epithelium of the diseased lung, a finding which is confirmed by the results of the present study demonstrating epithelial expression in the developing bronchial epithelium and in the alveoli during the canalicular period. This evidence concerns the gene CLDN5 and sarcoidosis.