Background: CD123 is an alpha chain of the interleukin-3 receptor, which belongs to the type I cytokine receptor family. CD123 forms a heterodimer with CD131, the interleukin-3β receptor, to form the interleukin-3 receptor, with an alpha subunit that dominates cytokine specificity and a beta subunit that dominates signaling. CD123 is expressed on monocytes, neutrophils, basophils, eosinophils, megakaryocytes, red blood cell precursors, mast cells, macrophages and some B cell subsets. It is often used in combination with CD68, CD43 and CD4 for the diagnosis of blastic plasmacytoid dendritic cell neoplasm (BPDC).