Previously, in two trials of conventional somatic gene therapy for a severe combined immunodeficiency (SCID), 5/20 subjects developed leukemia several years after the administration of CD34+ progenitor cells in which a retroviral vector encoding IL2RG was introduced.16,17 The side effect of leukemia was due to the activation of protooncogenes caused by the genomic insertion of retroviral vectors in an unintended manner. The gene discussed is IL2RG; the disease is leukemia.