Comparison of heteromeric cytokine receptor complexes. Cytokine receptor complexes for GM-CSF, IL-2, IL-4, IL-6, and IL-13. In all cases, the cytokine is shown in magenta, although the receptor subunits are colored according to the order in which they bind cytokine, red for the first subunit and blue or green for the second subunit, whereas the IL-2Rα subunit is yellow. Receptors are named, their domains are numbered D1 to D4, and the interacting surfaces are numbered I, II, and III. The inset panel at the bottom highlights the conserved interaction motif observed for GM-CSF/βc (site II), IL-2/IL-2Rα (site I), and IL-4/IL-4Rα (site I). Components are colored as above, and the key residues are shown in stick fashion, whereas cytokine helices A and C are labeled (Ha, Hc).