Long-term follow-up studies have demonstrated significant late toxicities of total body irradiation (TBI), which are most marked in children radiated at a young age. Growth failure, decline in cognitive function, and endocrine abnormalities have all been described. Good outcomes can be achieved with alkylating agents only as a preparative regimen. This plan will use a combination of busulfan and cyclophosphamide (Bu/Cy) with or without antithymocyte globulin (ATG) to reduce the late toxicities of therapy that includes TBI.

Inclusion Criteria:

• Suitable cord blood from matched unrelated or related donor.
• Cardiac (Echo/EKG): shortening fraction ≥ 27%
• Electrolytes within normal CCHMC limits.
• Pulmonary function tests: DLCO ≥ 50%
• Renal: creatinine clearance/GFR ≥ 60 ml/minute/1.73m2
• Lumbar puncture: no leukemic infiltrate.
• CBC: ANC ≥ 1000ml and unsupported platelet count of ≥ 50,000/ml
• Documented HSV and CMV titers, Hepatitis B surface antigen, Hepatitis C by serology, HIV by serology: all negative.
• Hepatic transaminases < 2.5x normal; Total bilirubin < 2 mg/dl Patients who do not meet above organ function criteria (liver, cardiac, renal), due to the presence of a tumor compromising these organs, may have exception made for these criteria and remain eligible for treatment plan after consultation with Program Director of Blood and Marrow Transplant

Exclusion Criteria:

• Patients with neoplastic or non-neoplastic disease of any major organ system that would compromise their ability to withstand the pre-transplant conditioning regimen.
• Patients with uncontrolled (culture or biopsy positive) infections requiring intravenous antivirals, antibiotics, or antifungals. Patients on prolonged antifungal therapy with a history of fungal infection should be considered for a non-myeloablative protocol.
• Patients who are pregnant or lactating. Patients of childbearing potential must practice an effective method of birth control while participating on this treatment plan.
• HIV seropositive patients