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]