Over 200 different mutations in γc have been identified in individuals with X-linked SCID.241–244 The structure of IL-2 in complex with IL-2Rα, IL-2Rβ, and γc is the first signaling complex structure solved for γc cytokines.245,246 The crystal structure reveals a quaternary complex consisting of one copy each of IL-2, IL-2Rα, IL-2Rβ, and γc (Fig. 5a). Here, GC is linked to T-B+ severe combined immunodeficiency due to gamma chain deficiency.