Mouse CD20 is a B-cell-specific transmembrane protein with a molecular weight of 33~37 kDa, also known as B-lymphocyte antigen, B1, and Bp35. It plays a crucial role in intracellular calcium regulation and B-cell activation, which is essential for optimal B-cell immune responses against T-independent antigens. During the transition of pre-B cells to immature B cells in the bone marrow, CD20 is first expressed under the combined induction of CD19 and IgM. Subsequently, as B cells mature, the expression level of CD20 gradually increases, and almost all mature B cells express a certain level of CD20. However, it is noteworthy that plasma cells or plasmablasts do not express CD20.