In the ABO blood group system any allele which does not give rise to A or B antigen expression is an O allele. The most common O alleles, O1 (O01) and O1v (O02) encode enzymatically inactive truncated proteins due to a single nucleotide polymorphism (SNP) compared to the consensus A1 allele, 261delG. Other O alleles containing the consensus nucleotide at residue 261 have been identified. The most common of these non-deletional O alleles is O2 (O03). It contains other SNPs which render the resulting protein non-functional. The most significant SNP is 802G>A (G268R); the arginine residue blocks the donor sugar’s access to the active site preventing its transfer. Recently O2 alleles were proposed to be the most common cause of ABO discrepancies (anti-A lacking or weakened in plasma despite apparent group O phenotype) in automated blood grouping (
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