The CD33 antigen is expressed widely on normal myeloid cells and has only moderate density on tumors (averaging 10,000 CD33 molecule per AML blast) leading to an unfavorable therapeutic index16,17,18 In addition, anti-CD45 RIT including [90Y]-labelled ([90Y]-anti-BC8) and [131I]-labeled [131I]-BC8 have been effective for pre-transplant conditioning by ablating both leukemic and normal hematopoietic cells but by design lead to severe hematologic toxicity, restricting use to fully myeloablative applications19,20. The gene discussed is CD33; the disease is acute myeloid leukemia.