Mesothelin, a glycosylphosphatidylinositol (GPI)-anchored cell surface glycoprotein. Its normal physiological expression is restricted mainly to mesothelial cells lining the pleura, peritoneum, and pericardium. In oncology, mesothelin is overexpressed in many aggressive cancers, including nearly all epithelioid mesotheliomas, about 70% of pancreatic adenocarcinomas, and subsets of ovarian and lung adenocarcinomas. Its overexpression is associated with worse prognosis. The biological function of mesothelin includes promoting cell adhesion, proliferation, and survival, partly through interaction with its binding partner CA125 (MUC16). This interaction may facilitate peritoneal metastasis.