SS1P is a RIT composed of a Fv that targets mesothelin and a 38-kDa fragment of Pseudomonas exotoxin A (PE38).SS1P was developed to treat a variety of mesothelin expressing tumors; these include mesothelioma, ovarian, pancreatic, lung, stomach and cervical cancer [1–4]. Here, MSLN is linked to mesothelioma.