Glycophorins A (GYPA) and B (GYPB) are major sialoglycoproteins of the human erythrocyte membrane which bear the antigenic determinants for the MN and Ss blood groups. GYPB gene consists of 5 exons and has 97% sequence homology with GYPA from the 5' UTR to the coding sequence encoding the first 45 amino acids. In addition to the M or N and S or s antigens, that commonly occur in all populations, about 40 related variant phenotypes have been identified.