H syndrome tissue biopsies contain RDD–like infiltrating histiocytes positive for phospho-ERK. (A) FFPE sections of skin lesion (patient H1, supplemental Table 1). Left: H&E staining demonstrates diffuse infiltration of the dermis by histiocytes with a large pale cytoplasm, accompanied by scattered lymphocytes and plasma cells. Middle and right: infiltrating histiocytes are positive for CD68 and S100. Emperipolesis is marked by an arrow. (B) The infiltrating histiocytes in skin lesion (left panel) and maxillary sinus tumor (right panel) of patient H1 show strong positivity for phospho-ERK. H&E, hematoxylin and eosin.