CD20 is a 33-37 KDa transmembrane glycoprotein involved in the regulation of B-cell activation, proliferation and differentiation. It is expressed on mature B-cells but not on immature B-cells or in plasma cells¹. It has been demonstrated weakly in only a small subpopulation of T-cells, but not in any other cell type. Only a minor portion of the CD20 antigen is located on the cell membrane; the majority is located within the cytoplasm, hence antibodies raised against this portion of the antigen, such as L26, are designated CD20cy². Along with CD79a, CD20 is one of the most commonly used B-cell markers in immunohistochemistry for the demonstration of B cell lymphomas such as follicular lymphoma, mantle cell lymphoma and chronic lymphocytic lymphoma. T-cell lymphomas are very rarely positive with CD20, although in Hodgkin’s lymphoma, CD20 stains up positive in the L&H cells in most cases of the nodular lymphocytic-predominant subtype, and Reed-Sternberg cells may also stain positive in a small proportion of tumour cells in other subtypes.