NCAM1 and Merkel cell skin cancer: Differential diagnoses of BPDCN include myeloid sarcoma, myelomonocytic leukemia (which can express CD4, CD56 and CD123), mature plasmacytoid dendritic cell proliferation (MPDCP) and Merkel cell carcinoma (MCC) which are all excluded in this case by appropriate immunohistochemistry study.