<h4>Introduction</h4>Chronic lymphocytic leukemia (CLL), the most common leukemia in Western countries, is a mature B-cell chronic lymphoproliferative disorder characterized by the accumulation of neoplastic CD5+ B lymphocytes, functionally incompetent and usually monoclonal in origin, in bone marrow, lymph nodes and blood. The gene discussed is CD5; the disease is B-cell chronic lymphocytic leukemia.