Inclusion Criteria:

1. Children > 3.5 months of age with a diagnosis of ADA-deficient SCID based on:

   • Confirmed absence (<3% of normal levels) of ADA enzymatic activity in peripheral blood or (for neonates) umbilical cord erythrocytes and/or leukocytes, or in cultured fetal cells derived from either chorionic villus biopsy or amniocentesis, prior to institution of enzyme replacement therapy.

   AND

   • Evidence of severe combined immunodeficiency based on either:

     • Family history of first order relative with ADA deficiency and clinical and laboratory evidence of severe immunologic deficiency,

   OR

   • Evidence of severe immunologic deficiency in subject based on lymphopenia (absolute lymphocyte count <200) or severely decreased T lymphocyte blastogenic responses to phytohemagglutinin (deltaCPM<5,000), prior to institution of immune restorative therapy.

   OR

   • Fulfillment of criterion:

     • A in addition to evidence of genetic mutations affecting the ADA gene as determined by a CLIA certified laboratory and clinical evidence of combined immunodeficiency based on lymphopenia (absolute lymphocyte counts <2SD of age-matched control values) and hypogammaglobulinemia (<2SD of age-matched control values) or lack of specific antibody response to vaccination. In addition, for patients to be eligible under this criterion, they must present with a clinical history indicating life-threatening illness characterized by increased frequency and/or severity of infections resulting in hospitalization and/or the administration of intravenous antibiotics, for bacterial or opportunistic infection.

2. Ineligible for allogeneic (matched sibling) bone marrow transplantation (BMT):

   • Absence of a medically eligible HLA-identical sibling with normal immune function who may serve as an allogeneic bone marrow donor.
3. Written informed consent according to guidelines of the Committee on Clinical Investigations (CCI) at Childrens Hospital Los Angeles (CHLA).

This study is also open to delayed/late onset ADA-deficient patients who fulfill the criteria 1, 2.A, and 3 and who are not receiving PEG-ADA treatment after being invited to discuss all alternative treatment options with a physician not connected with the protocol.

Exclusion Criteria:

1. Age less than 3.5 months.

2. Hematologic

   • Anemia (hemoglobin < 10.5 mg/dl at < 2 years of age, or < 11.5 at > 2 years of age,with normal serum iron studies).
   • Neutropenia (absolute granulocyte count <1,000/mm3 (6 mos - 1 year of age) or <1,500/mm3 (> 1 year of age).
   • Thrombocytopenia (platelet count 150,000/mm3, at any age).
   • PT or PTT >2X normal.
   • Cytogenetic abnormalities on peripheral blood.

3. Infectious

   • Evidence of active opportunistic infection or infection with HIV-1, hepatitis B, CMV or parvovirus B 19 by DNA PCR at time of assessment.

4. Pulmonary

   • Resting O2 saturation by pulse oximetry <95%.
   • Chest x-ray indicating active or progressive pulmonary disease.

5. Cardiac

   • Abnormal electrocardiogram (EKG) indicating cardiac pathology.
   • Uncorrected congenital cardiac malformation.
   • Active cardiac disease, including clinical evidence of congestive heart failure,cyanosis, hypotension.

6. Neurologic

   • Significant neurologic abnormality by examination.
   • Uncontrolled seizure disorder.

7. Renal

   • Renal insufficiency: serum creatinine > or = 1.2 mg/dl, or > or = 3+ proteinuria.
   • Abnormal serum sodium, potassium, calcium, magnesium, phosphate at grade III or IV by Division of AIDS Toxicity Scale.

8. Hepatic/GI:

   • Serum transaminases > 5X normal.
   • Serum bilirubin > 3.0 mg/dl.
   • Serum glucose > 250mg/dl.
   • Intractable severe diarrhea.

9. Oncologic

   • Evidence of active malignant disease.
10. Known sensitivity to Busulfan

11. General

    • Expected survival <6 months.
    • Pregnant.
    • Major congenital anomaly.
    • Medically eligible HLA-matched sibling.
    • Other conditions which in the opinion of the P.I. or co-investigators, contra-indicate infusion of transduced cells or indicate patient's inability to follow protocol.