One of the most important developments contributing to the understanding of the biology of histiocytic neoplasms was the detection of recurrent mutations of genes of the Ras/Raf/MEK/ERK signaling pathway, in particular, the BRAFV600E mutation in Langerhans cell histiocytosis (LCH) and Erdheim-Chester disease (ECD) [7, 8]. The gene discussed is MAP2K7; the disease is Langerhans cell histiocytosis.