IL3RA/CD123 is the alpha subunit of an IL-3 receptor that also contains the common beta chain (?c) shared with receptors for other cytokines. IL3RA/CD123 alone binds to IL-3 with high specificity but low affinity and it does not transduce intracellular signaling, while ?c alone does not bind to IL-3 appreciably. Together, IL3RA/CD123 and ?c form the functional high-affinity receptor for IL-3. IL3RA/CD123 is mainly expressed on the hematopoietic progenitor cells, primarily the granulocytic and monocytic linages . IL3RA/CD123 is highly expressed on leukemic stem cells in the majority of AML (acute myeloid leukemia) patients and is correlated with poor prognosis. In addition, overexpression of IL3RA/CD123 has also been reported for other hematopoietic malignancies. It is considered as a valuable therapeutic target for these malignancies.