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CD20, also known as B-lymphocyte antigen CD20, is a 33–37 kDa non-glycosylated protein primarily expressed on the surface of B cells from the pre-B cell stage through maturity, but it is absent on plasma cells. It is encoded by the MS4A1 gene and belongs to the membrane-spanning 4A gene family. CD20 is a transmembrane protein with four hydrophobic transmembrane domains, one intracellular domain, and two extracellular loops. Although its exact biological function remains unclear, it is believed to play a role in B cell activation and proliferation by facilitating calcium influx. CD20 is an important therapeutic target for B cell malignancies such as chronic lymphocytic leukemia and non-Hodgkin lymphoma, as well as for some autoimmune diseases. Anti-CD20 monoclonal antibodies, like Rituximab, work through mechanisms including complement-dependent cytotoxicity, antibody-dependent cell-mediated cytotoxicity, and hyper-crosslinking.