Mesothelin is a tumor differentiation antigen (40 kDa) that is normally present on the mesothelial cells of pleura, peritoneum, and pericardium (82, 83) and is highly expressed in many human cancers, including malignant mesothelioma, pancreatic, ovarian, and lung adenocarcinoma (84–87). The gene discussed is MSLN; the disease is cancer.