CD20 was distributed in all mature B cells, but plasma cells were mostly negative. CD20 is expressed in the vast majority of B-cell leukemia/lymphomas. Some myeloma and about 50% of B-cell lymphoblastic lymphomas are also positive for CD20, while T-cell lymphomas are almost negative. CD20 is the most commonly used B-cell marker and is mostly used to label B lymphocytes and B-cell lymphomas.