Recurrent gain-of-function BRAFV600E mutations are frequently found in LCH,31 while MAP2K1 mutations typically occur in BRAF wild-type lesions, consistent with broad ERK pathway activation in LCH.30,32 Other alterations affecting the Ras/Raf/MEK/ERK (MAPK) pathway are identified less frequently in patients lacking BRAF or MAP2K1 alterations. The gene discussed is MAP2K1; the disease is Langerhans cell histiocytosis.