The similarity of IL-21 to IL-2, IL-4, and IL-15, suggests the possibility that IL-21 might share an important receptor on the common cytokine receptor γ chain (γc), encoded by IL2RG, the gene found to be mutated in humans with X-linked severe combined immunodeficiency (XSCID) [45]. The gene discussed is IL4; the disease is T-B+ severe combined immunodeficiency due to gamma chain deficiency.