This protein is a minor sialoglycoprotein in erythrocyte membranes. The N-terminal extracellular domain is heavily glycosylated on serine and threonine residues. Along with GYPA, GYPB is responsible for the MNS blood group system. The molecular basis of the S/s blood group antigen is a single variation in position 48; Thr-48 corresponds to s=MSN4 and Met-48 to S=MNS3. Belongs to the glycophorin-A family. Blood group antigen gene mutation database