CD5 and B-cell chronic lymphocytic leukemia: CD5+ MBL is the most frequent, accounting for two thirds of all MBL, and is characterized by the presence of monoclonal B cells with a phenotype identical to CLL (CD20dim, CD5+, IGdim) in the peripheral blood, at concentrations lower than those required for the diagnosis of CLL (5 × 109/l) (13).