Toward a Molecular Genetic Understanding of Histiocytic Neoplasms A) Word cloud of the various names and eponyms for systemic histiocytic neoplasms that have been used since the initial descriptions of the diseases 150 years ago. B) Pie charts of somatic mutations affecting MAP kinase signaling that are now known to occur in Langherans cell histiocytosis and Erdheim-Chester disease. The majority of recurrent mutations that have been identified are mutually exclusive activating mutations affecting MAP kinase signaling, with the most common being the BRAF V600E mutation. Reprinted with permission from the American Association for Cancer Research.