This study investigated the safety and efficacy of different gene therapy approaches for Severe Combined Immunodeficiency (SCID) caused by the deficiency of adenosine deaminase (ADA) enzyme. This is a severe condition that can be cured by HLA-matched sibling donor bone marrow transplantation. Patients were enrolled if no HLA-identical sibling donor was available and the patient showed evidence of failure of enzyme replacement therapy or this treatment was not a long-term available option. The aim of the study was to evaluate the safety and efficacy of the procedure and to identify the relative role of peripheral blood lymphocytes and hematopoietic stem cells and progenitor cells in the long-term reconstitution of immune functions after retroviral vector mediated ADA gene transfer.