Up to 70 patients aged between 10 and 55 years will be studied in each subset (HLA 6/6 and 5/6 matched cohorts). The major endpoint of the study is acute GVHD after transplant. We will also measure engraftment, acute and chronic GVHD, leukemic relapse, transplant-related and all causes of mortality, cytomegalovirus reactivation and leukemia-free survival. Patients will be followed for a minimum of 5 years.