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.