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CD20 is a 33-37 kDa non-glycosylated phosphoprotein expressed on the surface of B cells from the pre-B cell stage through maturity, but absent on plasma cells. It belongs to the MS4A protein family and has four transmembrane domains, one intracellular domain, and two extracellular loops. CD20 is believed to facilitate calcium influx, which is essential for B cell activation and proliferation. Although its exact biological function is not fully elucidated, studies suggest it plays a role in B cell receptor signaling and may act as a calcium channel. CD20 is a key therapeutic target for B cell malignancies and autoimmune diseases, as it is expressed on various B cell cancers and its targeting can induce cell death through mechanisms like complement-dependent cytotoxicity and antibody-dependent cellular cytotoxicity.