ALK-positive histiocytosis can affect numerous anatomic sites, with bone marrow and liver being most common in Group 1A, and nervous system, liver, lung, and bone being most common in Group 1B and Group 2 disease subgroups. Adapted from Figure 6, Kemps PG et al. ALK-positive histiocytosis: a new clinicopathologic spectrum highlighting neurologic involvement and responses to ALK inhibition. Blood. 2022;139:256-280.