Hairy cell leukaemia (HCL) diagnosis is based on the morphologic detection of circulating abnormal hairy cells in the peripheral blood and/or bone marrow, an HCL immunological score of 3 or 4 based on the expression of the CD11c, CD25, CD103 and CD123 and also the presence of a BRAF V600E activating mutation in the B‐raf proto‐oncogene (BRAF gene) (7q34). The gene discussed is BRAF; the disease is hairy cell leukemia.