Immunohistochemical staining, performed on 4-μm thick formalin-fixed, paraffin-embedded tissue sections, showed that the histiocytes were positive for CD68 and CD163, and negative for S100, cyclin D1, CD1a, BRAF V600E, and ALK (Figure 1C and D) excluding diagnoses of Rosai-Dorfman disease (RDD), Langerhans cell histiocytosis (LCH), and ALK-positive histiocytosis. The gene discussed is CD1A; the disease is Histiocytosis.