Although a strong indicator of HCL, detection of the BRAF V600E mutation is not yet a mandatory criterion for the diagnosis of HCL, as rare cases otherwise consistent with HCL that use the IGHV4-34 family of immunoglobulin heavy chain and that have MAP2K1 mutations lack the BRAF V600E mutation [56,57]. The gene discussed is MAP2K1; the disease is hairy cell leukemia.