Although the effective half-life of whole IgG antibodies in the circulation has been considered to be longer than the physical half-life of 211At (7.2 h) as used in this study, in the case of AML, the reasonably slow clearance of 211At-CXCR4 mAb from blood may not be a disadvantage for the therapeutic effect on the stem cells as long as normal organ toxicity is tolerable, because AML stem cells may be present mainly in the bone marrow and partly in the circulation. This evidence concerns the gene CXCR4 and acute myeloid leukemia.