Although Tavor et al. (2004) found that homing of primary human AML cells to the bone marrow of non-obese diabetic (NOD)/severe combined immunodeficiency (SCID)/B2mnull mice is CXCR4 dependent, Monaco et al. (2004) reported that CXCR4 is dispensable for repopulation of human AML cells in NOD/SCID mice. The gene discussed is CXCR4; the disease is severe combined immunodeficiency.