Intermediate differentiation stage B-cell marker. Monoclonal antibody CAT13.6E12 recognizes the CD20 antigen. CD20 belongs to the MS4A family (membrane-spanning 4-domains subfamily A). Its expression is restricted to normal and neoplastic B-cells. It is expressed by pre-B-cells in the bone marrow, and expression persists during B-cell maturation but is lost upon terminal differentiation into plasma cells. CD20 is not glycosylated. CD20 is not phosphorylated in resting B-cells, but becomes heavily phosphorylated after mitogen stimulation. Three cell surface isoforms 33, 35 and 37kD, result from differential phosphorylation of a single core 33kD protein within the cytoplasmic domain. Multiple proteins are coimmunoprecipitated with CD20, suggesting that it may be a component of a multimeric surface receptor complex. Monoclonal antibodies against CD20 are used therapeutically, such as Rituximab (Mabthera) or Britumomab (Zevalin). Application: IHC (f), FACS