ALK-positive histiocytosis (APH), a rare non-Langerhans histiocytosis, was initially reported in 2008 and classified as a tumor category of histiocyte/macrophage in the 5th edition of the WHO Classification of Tumors of the Lymphatic and Hematopoietic System in 2022 (1). The gene discussed is ALK; the disease is Histiocytosis.