Allogeneic Stem Cell Transplant With Clofarabine, Ara-C and TBI for AML and ALL

Inclusion Criteria:

* Age: Patients must be \<30 years of age.
* Disease Status: ALL in relapse, induction failure, CR3, or CR3P (Part A ONLY); AML in relapse, induction failure, CR3, or CR3P (Part A ONLY); ALL in CR3 or CR3P (Part A and Part B); AML in CR3 or CR3P (Part A and BONLY); CR3/CR3P must be documented by bone marrow and CNS assessment within 14 days of initiation of the pre-transplant conditioning regimen.
* Creatinine clearance \>40 ml/min/m2 or \>60 ml/min/1.73 m2 or an equivalent radioisotope glomerular filtration rate (GFR) as determined by the institutional normal range or serum creatinine based on age
* Adequate liver function defined as: Total bilirubin \<2.5 mg/dl l, or SGOT (AST) or SGPT (ALT) \<5 x upper limit of normal
* Adequate cardiac function defined as: Shortening fraction \>27% by echocardiogram, or Ejection fraction of \>50% by radionuclide angiogram or echocardiogram.
* Adequate pulmonary function defined as: Corrected DLCO \>60% by pulmonary function test; For children who are uncooperative, no evidence of dyspnea at rest, no exercise intolerance, and a pulse oximetry \>94% on room air.
* Performance Status: For patients age 1-16 years, Lansky score of \>60; For patients \> 16 years, Karnofsky score of \>60.
* Patients must have received a minimum of one round of re-induction and one round of consolidation chemotherapy after relapse #2

Exclusion Criteria:

* Patients with prior myeloablative allogeneic stem cell transplantation and /or TBI.
* Females who are pregnant (positive HCG) or lactating.
* Karnofsky \<60% or Lansky \<60% if less than 16 years of age
* Age \>30 years of age
* Any patient with uncontrolled infection prior to study entry
* Patients with evidence of active disease.
* Patients with Down syndrome are excluded