The mesothelin gene encodes a 69-kDa precursor protein that is processed to a 40-kDa glycosylphosphatidylinositol (GPI)-anchored protein, the mature mesothelin, present on the cell surface. Its biological function is not known, but recent studies have shown that it forms a strong and specific complex with MUC16; a binding which has been suggested to be the basis of ovarian cancer metastasis. Mesothelin is present on normal mesothelial cells lining the pleura, peritoneum, and pericardium. In tumors, overexpression of Mesothelin has been observed in mesotheliomas, and other tumors including ovarian, pancreatic carcinomas, and cholangiocarcinoma. By using immunotoxin targeting immunotherapy, mesothelin has also been reported as a new therapeutic target in various types of cancers, such as human cholangiocarcinoma.