X-linked severe combined immunodeficiency disease (X-SCID), which leads to profound defects in the function of T-cell, B-cell, and natural killer (NK) cell functions, is caused by mutations in the interleukin 2 receptor γ-chain gene (IL2RG).1 The gene discussed is IL2RG; the disease is T-B+ severe combined immunodeficiency due to gamma chain deficiency.