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.