Gene therapy for ADA-deficient SCID could be performed by introducing a normal copy of the human ADA gene into the blood forming stem cells of the patient's bone marrow by using a new type of gene delivery system (in this trial called a lentiviral vector). The gene corrected cells are then transplanted back into the patient after small dose of chemotherapy. These gene corrected stem cells can survive into the body and make lymphocytes. In this trial, we will determine whether gene therapy for ADA-deficient SCID using lentiviral vector is safe, feasible and effective