The current protocol will utilize the combination of intravenous busulfan and etoposide. The busulfan dose will be escalated amongst 3 targeted dose levels (area under the curve (AUC) levels at time 6 hours of 1250 uMol*min, 1400 uMol*min and 1550 uMol*min). All targeted dose levels represent higher busulfan dosing than standard myeloablative dosing with the lowest dose (1250 uMol*min) being approximately 14% higher than standard. In the absence of dose-limiting toxicity, cohorts of 4-6 patients will be treated at each dose level and 10 additional patients will be treated at the maximum tolerated dose (MTD) to confirm safety. The busulfan dosing will begin at 1 mg/kg based on historical plasma levels obtained from patients receiving BU at a starting dose of 0.8 mg/kg at UCSF Medical Center.