Figure 1. The γ_c family of cytokines.

Shown are the receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21, as well as major actions for these cytokines. Crosses in red indicate that mutation of IL2RG gene, which encodes γ_c, results in X-linked severe combined immunodeficiency in humans (XSCID, where both T cells and NK cells are greatly diminished [T⁻B⁺NK⁻ SCID]), mutation of IL2RA results in an autoimmune syndrome, defective expression of IL2RB results in NK-deficient SCID (where T and B cells remain [T⁺B⁺NK⁻ SCID]), and mutation of IL7R causes T-cell selective form of SCID, where B and NK cell numbers are normal (T⁻B⁺NK⁺ SCID). JAK3 is not shown as it interacts with the cytoplasmic domain of γ_c; however, mutations in JAK3, as noted in the text, cause a T⁻B⁺NK⁻ for SCID, like XSCID.