Patients were diagnosed with diverse histiocytic neoplasms (Figure 1B; Table 1), including LCH (n = 12), ECD (n = 14), isolated CNS xanthogranuloma (CNS-XG; n = 2), Rosai–Dorfman disease (RDD; n = 3), HS (n = 2), ALK-positive histiocytosis (n = 1), mixed LCH/ECD (n = 3), mixed LCH/RDD (n = 1), mixed RDD/ECD (n = 1), and unclassifiable CNS histiocytosis (n = 1). This evidence concerns the gene ALK and Histiocytosis.