The diagnosis of RDD whilst not finite, is a composite of: (1) histological appearance of extensive sinusoidal expansion with sinusoids filled with numerous large histiocytes [1,4] and (2) the presence of histiocytes positive for S100, CD68 and CD163 and an absence of the cell markers seen in Erdheim-Chester disease (ECD), Juvenile xanthogranuloma (JXG) or Langerhans cell histiocytosis (LCH) [5]. The gene discussed is CD163; the disease is Langerhans cell histiocytosis.