Three main phenotypic features define the CLL B cell: (a) the predominant population shares B-cell markers (CD19, CD20 and CD23) with the CD5 antigen, in the absence of other pan-T-cell markers; (b) the B cells are monoclonal with regard to expression of either κ or λ light chains; and (c) express surface immunoglobulin, CD79β, CD20 and CD22 with low density. Here, CD5 is linked to B-cell chronic lymphocytic leukemia.