Morphologic evaluation and immunohistochemical markers that aid in the diagnosis of BPDCN. (A) Skin lesions of BPDCN can vary in shape, size, color, and distribution. Hyperpigmented red-brown macules, shown here, may be confused with neoplastic and nonneoplastic etiologies. (B) Skin biopsy (×500) of dermal infiltrate of immature mononuclear cells, which spares the epidermis (separated by a Grenz zone) typical of BPDCN, LC, and myeloid sarcoma (MS) and helps distinguish those diseases from mycosis fungoides which is usually epidermotropic. (C) Bone marrow aspirate (×1000) demonstrates medium to large cells with scant cytoplasm, immature chromatin, irregular nuclear contours, and prominent nucleoli. (D) Shared immunohistochemical markers are shown with a range of positive cases observed for BPDCN and AML/LC/MS. Ranges are rounded to the nearest 5% based on multiple series.^2,17,18,26  Clearly the overlap of shared markers and exception of atypical cases that lack a particular marker highlight the need for review of unique markers to differentiate BPDCN from AML/LC/MS.