CD20 is a 33-37 kDa transmembrane phosphoprotein. CD20 is a B-lymphocyte surface molecule that is widely expressed during B-cell ontogeny, from early pre-B-cell developmental stages until final differentiation into plasma cells. CD20 functions as calcium-permeable cation channel. It is involved in the regulation of B-cell activation and proliferation. CD20 serves as a useful target for antibody-mediated therapeutic depletion of B-cells. This antibody is conjugated with FY488.