CD20 is a transmembrane, non-glycosylated protein expressed on B-cell precursors and mature B cells, but is lost following differentiation into plasma cells. In resting B cells, CD20 appears in a 33 kDa non-phosphorylated form. After mitogen stimulation, CD20 becomes heavily phosphorylated (35-37 kDa isoforms), and it is a dominant phosphoprotein in activated B cells. The antibody reacts with an intracytoplasmic epitope localized on the CD20 antigen and labels cells of the B-cell lineage. It is a useful aid for classification of neoplasms of B-cell derivation.