About 11 samples of blood (about 1 teaspoon each time) will be drawn for pharmacokinetic (PK) testing of busulfan. PK testing measures the amount of study drug in the body at different time points and will help the study doctor determine what your dose of busulfan should be on study. These blood samples will be drawn at various timepoints before you receive busulfan and over about the next 11 hours. The blood samples will be repeated again on the first day of high-dose busulfan treatment (Day -8). A temporary heparin lock line will be placed in your vein to lower the number of needle sticks needed for these draws. If it is not possible for the PK tests to be performed for technical or scheduling reasons, you will receive the standard fixed dose of busulfan.