One approach is the targeting of ALL blasts with CAT-8015 (HA22, Moxetumomab pasudotox), a second-generation, high affinity recombinant immunotoxin (IT) composed of a 38 kDa fragment of Pseudomonas exotoxin A (PE38) fused to the disulphide-linked variable fragment of the murine anti-CD22 monoclonal antibody RFB4, which replaces the toxin's native cell binding domain I to specifically target CD22 antigen on the surface of ALL lymphoblasts (4–7). This evidence concerns the gene CD22 and acute lymphoblastic leukemia.