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Mouse CD20, similar to its human counterpart, is a B cell-specific surface antigen that plays a role in B cell activation and is involved in the regulation of transmembrane calcium conductance. It is first expressed during the late pre-B cell stage of development and its expression increases as B cells mature in the bone marrow. CD20 expression is initiated at the transition from pre-B cells to immature B cells and continues to be expressed on mature B cells, with the exception of plasma cells. In mice, CD20 expression is B cell restricted and can be used as a marker for certain B cell subsets, including transitional T1 B cells in the spleen, which also express high levels of CD20. This makes CD20 a useful marker for studying these cells. The frequency and density of CD20 increase during B cell maturation, with a subpopulation of transitional IgMhi B cells showing higher CD20 levels than most mature recirculating B cells.