Other histiocytic lesions, including Erdheim-Chester disease (ECD), Rosai-Dorfman-Destombes disease (RDD), cutaneous and mucocutaneous non-Langerhans cell histiocytoses, malignant histiocytoses (MH), hemophagocytic lymphohistiocytosis (HLH), and ALK-positive histiocytosis can show overlapping histological and clinical features, making differential diagnosis a significant challenge for pathologists. This evidence concerns the gene ALK and Histiocytosis.