Related to X-linked severe combined immunodeficiency (SCID-X1) caused by mutations in the interleukin-2 receptor γ chain gene (IL2RG), a self-inactivating lentivirus vector expressing a codon-optimized human IL2RG gene restored T, B, and NK cell counts in bone marrow and peripheral blood, and normalized thymus and spleen cellularity and architecture in a mouse model [72]. Here, IL2RG is linked to T-B+ severe combined immunodeficiency due to gamma chain deficiency.