2. The initial dose of busulfan (Dose cohort #1 = 1 mg/kg, Dose cohort #2 = 1.2 mg/kg, Dose cohort # 3 = 1.4) will be given as a single intravenous dose on Day -10 (in the morning, at 9:00AM). The dose will be administered by intravenous injection over 2 hours in the Outpatient Ambulatory Care/Infusion Center.