Myeloid sarcoma expressing cytoplasmic NPM1 and flow cytometric detection of cytoplasmic nucleophosmin in AML. (A) Multiple skin lesions; the arrow indicates the largest lesion. (B) Leukemic cells infiltrating the derma show aberrant cytoplasmic expression of NPM (arrow); the cells of epidermis (e) exhibit the expected nucleus-restricted positivity for NPM. (C) Leukemic cells express the histiocyte-restricted form of CD68 (monoclonal antibody PG-M1). (D) Leukemic cells are CD34⁻; the arrow indicates a CD34⁺ vessel that serves as internal control. (B-D) Alkaline phosphatase antialkaline phosphatase technique; hematoxylin counterstaining; images were collected using an Olympus B61 microscope and a UPlan FI 100×/1.3 NA oil objective; Camedia 4040, Dp_soft Version 3.2; and Adobe Photoshop 7.0. (E) Flow cytometry analysis of cytoplasmic nucleophosmin in AML. NPM1-mutated AML M5b 48% blasts showing the phenotype: CD34⁻CD13⁺CD33⁺CD117⁺MPO⁻ CD56⁺NPMc⁺. (F) AML M1 with wild-type NPM1 gene and 93% blasts with phenotype: CD34⁺ CD13⁺ CD33⁺ CD117⁺ MPO⁺CD56⁺NPMc⁻ (bottom left and right; courtesy of Prof Christian Thiede and Dr U. Oelschlaegel, University of Dresden, Dresden, Germany).