The receptor for IL-3 is made up of two subunits, both members of the cytokine receptor superfamily. The smaller ♂ subunit has a low affinity for IL-3 and the complex of IL-3 and the ♂ subunit binds to the ♀ subunit to form a high-affinity signaling complex. The ♀ subunit is shared with the receptors for GM-CSF and IL-5, hence its name, ♀ common (♀c), and alone, has no affinity for any of these three cytokines. In the mouse, however, there is an additional, IL-3- specific ♀ subunit, which is encoded by a duplication of the ♀c gene and which has itself a low affinity for IL-3. Signaling downstream of the receptor complex involves activation of JAK2 kinase, tyrosine phosphorylation of ♀c and the initiation of signaling paths regulating growth, survival, and differentiation.