CD34 and acute myeloid leukemia: Clinical trials performed using purified CD34+ cells have confirmed that sustained engraftment of donor hematopoiesis, without the occurrence of GvHD, can be obtained in the majority of adult patients and that a substantial proportion of them, especially when affected by acute myeloid leukemia (AML) or myelodysplastic syndromes, become long-term survivors (Aversa et al., 1998; Ruggeri et al., 2002).