On either Day -11 or -10, blood (about 1 teaspoon each time) will be drawn for pharmacokinetic (PK) testing 11 times over the 11 hours after your test dose 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. The PK blood draws 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.