CD34 and Langerhans cell histiocytosis: In patients with systemic LCH and/or ECD, the BRAFV600E mutation can be identified in CD34+ hematopoietic stem/progenitor cells (HSPCs) and their offspring.10, 11, 12, 13, 14 Moreover, mutated bone marrow cells of patients with LCH or ECD formed histiocytic lesions in xenograft mouse models.12