X-linked Severe Combined Immunodeficiency (X-SCID), characterized by mutations in the IL2RG gene and resulting in impaired humoral and cell immunity [13], was effectively corrected by the HDR-mediated editing of IL2RG via ZFNs, for both in patient-derived HSCs and xenotransplantation mouse models [14,15,16]. The gene discussed is IL2RG; the disease is T-B+ severe combined immunodeficiency due to gamma chain deficiency.