You will receive a dose of busulfan by vein over about 3 hours on Day -13 and Day -12. With the Day -13 busulfan infusion, about 11 samples of blood (about 1-2 teaspoons each time) will be drawn for pharmacokinetic (PK) testing at various time points before and after you receive your first dose of busulfan. The study staff will tell you the blood testing schedule. PK testing measures the amount of study drug in the body at different time points. The PK testing will help the doctor decide your dose of busulfan for Days -6 through -3. If needed, PK blood testing may also be done on Day -6 during your dose of Busulfan. You may receive the Day -13 and Day -12 busulfan dose either as an outpatient in the clinic or as an inpatient in the hospital.