Mesothelin (MSLN) is a cell surface-bound, glycosylphosphatidylinositol (GPI) anchored protein, whose normal expression is restricted to the mesothelial cells of the pleura, pericardium, peritoneum, and tunica vaginalis in men, whereas very low MSLN expression (trace amounts) has also been observed in the epithelial lining of the ovaries, fallopian tubes, and rete testis. Mesothelin (MSLN) is a tumor differentiation antigen normally restricted to the body’s mesothelial surfaces, but significantly overexpressed in a broad range of solid tumors. For this reason, MSLN has emerged as an important target for the development of novel immunotherapies.