The cell-surface antigen CD20 is expressed on more than 90% of B-cell lymphomas and chronic lymphocytic leukemias, and on 50% of pre-B-cell acute lymphoblastic leukemia. It is expressed on normal B cells from the pre-B-cell stage to the activated B-cell stage, but is not expressed on stem cells, plasma cells, or cells of other lineages. It is a transmembrane protein that appears to act as a calcium channel and to play an important role in cell-cycle progression and differentiation. Monoclonal antibodies that target the CD20 antigen have been developed, both for diagnostic and therapeutic purposes. The CD20 antigen is appealing for targeted therapy, because it does not shed, modulate, or internalize.
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