CD20 (also known as MS4A1) is a 297 amino acid (aa) non-glycosylated type III membrane protein that is the founding member of the MS4A tetraspanning protein family. CD20 is expressed on pre-B, naïve and mature B lymphocytes and B-cell lymphomas. CD20 is associated with lipid rafts upon crosslinking, and appears to play a role in regulation of ion influx. The one extracellular segment of 48 aa shows 65% aa identity between human and mouse CD20.