ALK-positive histiocytosis is a rare subtype of histiocytic neoplasm that was first described in 2008 in three neonates and is now included as a new entity in the fifth edition of the World Health Organization Classification of Haematolymphoid Tumors: Myeloid and Histiocytic/Dendritic Neoplasms.4 The gene discussed is ALK; the disease is Histiocytosis.