Interleukin-2 (IL-2), a cytokine produced by activated T cells, promotes the proliferation, differentiation, and survival of mature T and B cells. Its actions are primarily mediated through a quaternary signaling complex that consists of IL-2, the α and β receptors (IL-2Rα, IL-2Rβ), and the γc chain. Now Wang et al. present the extracellular structure of the quaternary complex at 2.3 angstroms resolution. Besides providing insight into IL-2 interactions that might facilitate design of IL-2 agonists and inhibitors, the structure provides a view of the γc receptor. This receptor is shared for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21 and is mutated in X-linked severe combined immunodeficiency diseases (X-SCID). Several mutations associated with X-SCID map to residues in the γc binding sites.