Often preceded by benign monoclonal B-cell lymphocytosis (MBL), defined as the presence of a clonal B-cell population in the peripheral blood with less than 5 × 109/L B-cells with a CLL immunophenotype (mature CD5+ cells) and no other signs of a lymphoproliferative disorder [25], it then transforms into leukemic disease. The gene discussed is CD5; the disease is B-cell chronic lymphocytic leukemia.