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a complex blood group system consisting principally of two pairs of antigens determined by closely linked genes (crossovers have been observed, but rarely). M and N, determined by allelic genes, depend on sialic (neuraminic) acid residues. S and s are also determined by allelic genes, and an amorphic gene is common in blacks when another antigen (U) is missing. The system also includes numerous low frequency antigens.Medical dictionary. 2011.