Human mesothelin, also known as CAK1 and MSLN, is a cell-surface protein that is expressed on normal mesothelial cells lining the pleura, pericardium, and peritoneum. It is expressed as a 70 kD pre-cursor protein with a furin cleavage site, which produces two products: 1) megakaryocyte potentiating factor (MPF) (30 kD) and 2) mesothelin (40 kD). MPF is a cytokine that is released. Mesothelin, for the most part, is membrane-bound by a glycosylphosphatidylinositol linkage and has three glycosylation sites. Soluble mesothelin is believed to result from proteolytic cleavage to release it from the membrane linkage, or from alternative splicing of the mesothelin gene.