Mesothelin (MSLN) is a 40kDa GPI-anchored surface glycoprotein that is expressed on pleural and peritoneal mesothelial cells, but not in other non-neoplastic tissues. The mesothelin gene encodes a 69-71kDa precursor protein that is processed to a 40kDa membrane-bound protein termed mesothelin and a 31kDa shed fragment called megakaryocyte-potentiating factor (MPF) that is released from the cell. Mesothelin is a tumor differentiation antigen present at low levels on a restricted set of normal adult tissues (e.g. mesothelium), and is expressed at high levels in mesotheliomas, as well as ovarian, pancreatic, and lung cancers. Mesothelin may be a new target for immunotherapy (PMID: 20922056, 18691948, 15217923).