The interleukin-2 (IL-2) receptor γ chain (IL-2Rγ) is an essential component of high- and intermediate-affinity IL-2 receptors. IL-2Rγ was demonstrated to be a component of the IL-4 receptor on the basis of chemical cross-linking data, the ability of IL-2Rγ to augment IL-4 binding affinity, and the requirement for IL-2Rγ in IL-4-mediated phosphorylation of insulin receptor substrate-1. The observation that IL-2Rγ is a functional component of the IL-4 receptor, together with the finding that IL-2Rγ associates with the IL-7 receptor, begins to elucidate why deficiency of this common γ chain (gamma; c ) has a profound effect on lymphoid function and development, as seen in X-linked severe combined immunodeficiency.