FOLR2 and hairy cell leukemia: Several immunotoxins consisting of mAbs conjugated to toxins have been used for clinical trials, such as in acute lymphoblastic leukemia,32 hairy cell leukemia,33 and mesothelioma.34 The truncated Pseudomonas aeruginosa exotoxin known as PE38 lacks its binding domain.35 Our recombinant dsFv anti–FRβ-PE38 immunotoxin consisted of the truncated PE38 and the heavy- and light-chain portions of the anti-FRβ antibody.