Mesotheliomas are malignant tumors with an aggressive behavior. In pleural biopsies taken for malignant pleural disease, there is often diagnostic difficulty to distinguish mesotheliomas from metastatic adenocarcinomas or reactive mesothelial cells. Several markers, such as calretinin, DJ2-40, TTF1 and CEA, can be used in differential diagnosis [106] . Claudins have also proven to be one putative means of making a differential diagnosis between such entities [107] [108] [109] [110] .