Interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21 form a family of cytokines based on the sharing of a receptor component, the common cytokine receptor γ chain, γc, which is encoded by the gene mutated in humans with X-linked severe combined immunodeficiency (XSCID). The gene discussed is IL21; the disease is T-B+ severe combined immunodeficiency due to gamma chain deficiency.