In this context, Claudin 4 was identified to be an important marker for carcinoma vs. mesothelioma diagnosis in pleural and peritoneal biopsies and effusions, as it is detected in metastatic tumor cells but not in benign forms of mesotheliomas (Facchetti et al., 2007; Lonardi et al., 2011). The gene discussed is CLDN4; the disease is mesothelioma.