This is a Phase I/II non-randomized clinical trial of ex vivo hematopoietic stem cell (HSC) gene transfer treatment for X-linked severe combined immunodeficiency (XSCID, also known as SCID-X1) using a self-inactivating lentiviral vector incorporating additional features to improve safety and performance. The study will treat 35 patients with XSCID who are between 2 and 50 years of age and who have clinically significant impairment of immunity. Patients will receive a total busulfan dose of approximately 6 mg/kg/body weight (target busulfan Area Under Curve is 4500 min\*micromol/L/day) delivered as 3mg/kg body weight on day 1 and dose adjusted on day 2 (if busulfan AUC result is available) to achieve the target dose, to condition their bone marrow, and this will be followed by a single infusion of autologous transduced CD34+HSC. Patients will then be followed to evaluate engraftment, expansion, and function of gene corrected lymphocytes that arise from the transplant; to evaluate improvement in laboratory measures of immune function; to evaluate any clinical benefit that accrues from the treatment; and to evaluate the safety of this treatment. The primary endpoint of the study with respect to these outcomes will be at 2 years, though data relevant to these measures will be collected at intervals throughout the study and during the longer follow-up period of at least 15 years recommended by the Food and Drug Administration (FDA) Guidance "Long Term Follow-Up After Administration of Human Gene Therapy Products" https://www.fda.gov/media/113768/download for patients participating in gene transfer clinical trials. XSCID results from defects in the IL2RGgene encoding the common gamma chain (yc) shared by receptors for Interleukin 2 (IL-2), IL-4, IL-7, IL-9, IL-15 and IL-21. At birth XSCID patients generally lack or have a severe deficiency of T-lymphocytes and NK cells, while their B- lymphocytes are normal in number but are severely deficient in function, failing to make essential antibodies. The severe deficiency form of XSCID is fatal in infancy without intervention to restore some level of immune function. The best current therapy is a T-lymphocyte-depleted bone marrow transplant from an HLA tissue typing matched sibling, and with this type of donor it is not required to administer chemotherapy or radiation conditioning of the patient's marrow to achieve excellent engraftment and immune correction of an XSCID patient. However, the great majority of patients with XSCID lack a matched sibling donor, and in these patients the standard of care is to perform a transplant of T- lymphocyte depleted bone marrow from a parent. This type of transplant is called haploidentical because in general a parent will be only half- matched by HLA tissue typing to the affected child. Whether or not any conditioning is used, haploidentical transplant for XSCID has a significantly poorer prognosis than a matched sibling donor transplant. Following haploidentical transplant, XSCID patients are observed to achieve a wide range of partial immune reconstitution and that reconstitution can wane over time in some patients. That subset of XSCID patients who either fail to engraft, fail to achieve adequate immune reconstitution, or lose immune function over time suffer from recurrent viral, bacterial and fungal infections, problems with allo- or autoimmunity, impaired pulmonary function and/or significant growth failure. We propose to offer gene transfer treatment to XSCID patients\^3 \>= 2 years of age who have clinically significant defects of immunity despite prior haploidentical hematopoietic stem cell transplant, and who lack an HLA-matched sibling donor. Our current gene transfer treatment protocol can be regarded as a salvage/rescue protocol. Prior successful retroviral gene transfer treatment instead of bone marrow transplant (BMT) in Paris and London for 20 infants with XSCID has provided proof of principle for efficacy. However, a major safety concern is the occurrence of 5 cases of leukemia at 3-5 years after treatment triggered in part by vector insertional mutagenesis activation of LMO2 and other DNA regulatory genes by the strong enhancer present in the long-terminal repeat (LTR) of the Moloney Leukemia Virus (MLV)- based vector. Furthermore, previous studies of gene transfer treatment of older XSCID patients with MLV- based vectors demonstrated the additional problem of failure of adequate expansion of gene corrected T- lymphocytes to the very high levels seen in infants. To reduce or eliminate this leukemia risk, and possibly enhance performance sufficiently to achieve benefit in older XSCID patients, we have generated a lentivector with improved safety and performance features. We have generated a self-inactivating (SIN) lentiviral vector that is devoid of all viral transcription elements; that contains a short form of the human elongation factor 1a (EF1a) internal promoter to expres...