Flowchart illustrating the phenotypic diagnosis of BPDCN. In the absence of expression of specific lineage markers, high expression of CD123 and HLA-DR plus CD4 (which can be low) and CD56 (which can be low or negative) raises the possibility of BPDCN, even if isolated or associated less-specific lineage markers are expressed (such as CD7, C2, CD33, CD13, CD117, CD22, and cCD79a). Diagnosis should be confirmed using cTCL1, CD303, and CD304. c, intracytoplasmic; −, negative (expressed in <20%of the blastic population); +, positive (>20%); low, intensity of expression less than normal cells expressing this marker (eg, NK cells for CD56, normal pDCs for CD303 and CD304, T cells for CD4, and monocytes for CD11c and CD64).