Morphology and immunohistochemistry of biopsies obtained from the bone marrow, skin, and a supraclavicular lymph node showed an infiltration of rather large, pleomorphic malignant cells (Fig. 1A) with a high proliferation (Ki-67: 80%) and histiocytic differentiation (CD14+, CD68+, and CD163+, supplementary Table 3), consistent with a diagnosis of histiocytic sarcoma (HS). The gene discussed is MKI67; the disease is histiocytic sarcoma.