TERT and Histiocytosis: Histologically, it was composed of epithelioid histiocytic cells and foamy cells, accompanied by the most common gene alteration of the KIF5B-ALK gene fusion in ALK-positive histiocytosis, and without BRAF-V600E, TERT, IDH1, IDH2, and MGMT gene mutations, which could be easily distinguished from ECD syndrome.