With substantial progress in therapeutic protein deimmunization by the Pastan group, immunotoxins (ITs) such as moxetumomab pasudotox (composed of an anti-CD22 antibody genetically fused to PE38) and SS1P (an anti-mesothelin immunotoxin), are now showing durable complete responses and major tumor regressions in aggressive diseases such as relapsed/refractory hairy cell leukemia (HCL) and mesothelioma respectively [30–32]. Here, MSLN is linked to hairy cell leukemia.