Mesothelin is derived from a 70 kDa precursor that also includes Megakaryocyte Potentiating Factor (MPF).The 70 kDa precursor is expressed on the cell surface where it is cleaved at a dibasic proteolytic site to release the 32 kDa glycosylated MPF. MPF is a cytokine that potentiates IL3 induced megakaryocyte colony formation. The term Mesothelin refers to the 40 kDa glycosylated protein which remains attached to the cell surface via a GPI linkage. Mesothelin is over expressed in several human tumors, including mesothelioma and ovarian and pancreatic adenocarcinoma. The interaction between mesothelin and MUC16 (also known as CA125) may facilitate the implantation and peritoneal spread of tumors by cell adhesion. The region (296-359) consisting of 64 amino acids at the N-terminal of cell surface mesothelin is the functional binding domain for MUC16. Mesothelin is a tumour differentiation antigen that is normally present on the mesothelial cells lining the pleura, peritoneum and pericardium. A soluble form of Mesothelin, with its GPI anchor intact, is released from tumor cells and binds to MMR/CD206 on macrophages.