MAPK1 and Langerhans cell histiocytosis: Since the discovery of recurrent MAPK pathway mutations in LCH and the confirmation of MAPK pathway activation (ERK phosphorylation) in LCH lesions [5–7], MAPK pathway inhibitors, which have been used in the treatment of other tumors, such as melanoma, have been used to treat histiocytes in patients and early-phase trials.