According to the consensus guidelines, a diagnosis of HCL is suggested by the clinical presentation of the patient and confirmed by laboratory findings including a complete blood count with peripheral blood smear review, a bone marrow aspiration, and trephine biopsy with the assessment of bone marrow morphology, immunohistochemistry, flow cytometry, and testing for the identification of the BRAF V600E mutation. This evidence concerns the gene BRAF and hairy cell leukemia.