CD123 is a 70 kD alpha chain subunit of the IL-3 receptor (IL-3R alpha). It is a member of the immunoglobulin superfamily that is expressed on hematopoietic progenitors, basophils, mast cells, and megakaryocytes. This transmembrane glycoprotein can bind IL-3 with low affinity but cannot transduce signals without association with additional protein partners. CD123 can complex with either the common beta chain (CDw131) or the IL-3R beta chain (AIC2A) to form high-affinity heterodimeric IL-3 receptors. CDw131 can complex with the alpha subunits of the mouse IL-3R, IL-5R and GM-CSFR to form high-affinity receptors, while the IL-3 R beta subunit is specific for IL-3 but binds with low affinity. IL-3 binding to the receptor complex can induce proliferation and differentiation of hematopoietic cells.