CD20 is a non glycosylated protein with a molecular weight of 35 or 37 kDa depending on the degree of phosphorylation. Although not a member of the tetraspanin superfamily of cell surface receptors, it crosses the cell membrane four times. The CD20 antigen is present on human pre B lymphocytes and on B lymphocytes at all stages of maturation, except on plasma cells. Low level expression of the CD20 antigen has been detected on normal T lymphocytes. The CD20 molecule is involved in regulation of B cell differentiation, presumably via its reported function as a Ca2+ channel subunit.