The GYPB gene encodes the protein Glycophorin B, a component of the MNS blood group. The GYPB gene consists of five exons that span more than 58 kbp.
The GYPB gene has two codominant alleles ('C' and 'c') called MNS3 and MNS4, which produce the S antigen and the s antigen, respectively, differing in one amino acid. The MNS3 allele encodes a methionine at residue 29, whereas the MNS4 allele encodes a threonine at this position.
The SNP that assays the GYPB gene S/s status is rs7683365. As represented on the forward (plus) strand, the rs7683365(A) allele determines the methionine and thus the S antigen, while the rs7683365(G) leads to the threonine and therefore the s antigen.[PMID 21257350]