X-linked severe combined immunodeficiency (SCID-X1 or XSCID) is caused by mutations in IL2RG gene which encodes the common cytokine gamma chain (γc) subunit shared by multiple cytokine receptors (interleukin (IL)-2, 4, 7, 9, 15 and 21). This evidence concerns the gene IL2RG and T-B+ severe combined immunodeficiency due to gamma chain deficiency.