The interleukin-2 receptor γ chain (IL-2Rγ) is a necessary component of functional IL-2 receptors. IL-2Rγ mutations result in X-linked severe combined immunodeficiency (XSCID) in humans, a disease characterized by the presence of few or no T cells. In contrast, SCID patients with IL-2 deficiency and IL-2-deficient mice have normal numbers of T cells, suggesting that IL-2Rγ is part of more than one cytokine receptor. By using chemical cross-linking, IL-2Rγ was shown to be physically associated with the IL-7 receptor. The presence of IL-2Rγ augmented both IL-7 binding affinity and the efficiency of internalization of IL-7. These findings may help explain the defects of XSCID. Given its role in more than one cytokine receptor system, the common γ chain (gamma; c ) is proposed as the designation for IL-2Rγ.