Severe combined immunodeficiency disorder (SCID) is a heterogeneous group of inherited disorders characterized by a profound reduction or absence of T lymphocyte function, resulting in lack of both cellular and humoral immunity. SCID arises from a variety of molecular defects which affect lymphocyte development and function. The most common form of SCID is an X-linked form (SCID-X1), which accounts for 30-50% of all cases. SCID-X1 is caused by defects in the common cytokine receptor gamma chain, which was originally identified as a component of the high affinity interleukin-2 receptor (IL2RG). Allogeneic haematopoietic stem cell transplantation (HSCT), which replaces the patient's bone marrow with that of a healthy donor, is the only treatment that definitively restores the normal function of the bone marrow. HSCT is the first choice of treatment for patients with signs of bone marrow failure and a fully-matched related donor. However, patients without a fully-matched related donor have much worse overall outcomes from HSCT. This study will investigate whether patients with SCID-X1 without a fully matched related donor may benefit from gene therapy. To do this the investigators propose to perform a phase I/II clinical trial to evaluate the safety and efficacy (effect) of gene therapy for SCID-X1 patients using a lentivirus delivery system containing the IL2RG gene. Up to 5 eligible SCID-X1 patients will undergo mobilisation and harvest of their haematopoietic stem precursor cells (HPSCs). In the laboratory the disabled lentivirus will be used to insert a normal human IL2RG gene into the patient's harvested HPSCs. Patients will receive chemotherapy conditioning prior to cell infusion, in order to enhance grafting. The genetically corrected stem cells will then be re-infused into the patient. Patients will be followed up for 2 years. This trial will determine whether gene therapy for SCID-X1 using a lentiviral vector is safe, feasible and effective