IL2RG and leukemia: Moloney murine leukemia virus-based gamma retroviral vectors (γRVs) carrying wild-type IL2RG sequences result in good long-term T cell immune reconstitution and an overall survival rate of 90% (18/20) 7, 8, 9; however, six of the patients developed T cell acute lymphoblastic leukemia 2–14 years after undergoing gene therapy.10, 11, 12, 13, 14, 15 Consequently, a new generation of SIN γRVs and SIN-LVs was developed, and none of the patients treated with these vectors developed leukemia.22