To date, three immunotoxins have been FDA approved to be used in humans; interleukin -2 conjugated to the diphtheria toxins (OntakTM) administered as an antineoplastic agent, interleukin -3 conjugated to the diphtheria toxin (ElzonrisTM) used for the treatment of blastic plasmacytoid dendritic cell neoplasms (29) and anti-CD22 conjugated to Pseudomonas exotoxin A (Lumoxiti®) approved for the treatment of relapsed or refractory hairy cell leukemia (30). This evidence concerns the gene IL2 and hairy cell leukemia.