CD20 is a transmembrane protein in late B cell precursors and mature B cells that plays a role in the regulation of proliferation and differentiation. CD20 expression is lost at the stage of plasma cell differentiation. Anti-CD20 (pan-B cell) has rarely been detected in T-cell malignancies and is a reliable marker for B-cell lymphomas such as DLBCL. CD20 expression is present in some thymomas.