CD123, also known as Interleukin-3 receptor subunit alpha, belongs to the type I cytokine receptor family. In mouse, there are two classes of high-affinity IL3 receptors. One contains an IL3-specific beta subunit and the other contains the beta subunit also shared by high-affinity IL5 and GM-CSF receptors. CD123 stimulates the proliferation and differentiation of hemopoietic cells including the pluripotent hematopoietic stem cells as well as various lineage-committed cells. CD123 is a heterodimer consisting of an alpha and a beta subunit. The alpha subunit alone binds IL-3 with low affinity. The beta subunit does not bind IL-3 by itself but is required for the high-affinity binding of IL-3 to the heterodimeric receptor complex.