ALK and Histiocytosis: The cohort comprised of 6 (15%) children and 34 (85%) adults with diverse histiocytoses, including Langerhans cell histiocytosis (LCH; n = 12), Erdheim–Chester disease (n = 14), central nervous system xanthogranuloma (n = 2), Rosai–Dorfman disease (n = 3), histiocytic sarcoma (n = 2), ALK–positive histiocytosis (n = 1), and mixed/unclassifiable histiocytosis (n = 6).