The interleukin-2 receptor ``gamma'' chain (IL-2R``gamma'') is a necessary component of functional IL-2 receptors. IL-2R``gamma'' 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-2R``gamma'' is part of more than one cytokine receptor. By using chemical cross-linking, IL-2R``gamma'' was shown to be physically associated with the IL-7 receptor. The presence of IL-2R``gamma'' 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 ``gamma'' chain (``gamma''c) is proposed as the designation for IL-2R``gamma''.