We have encountered an allele which seems to be another type of O allele at the human histo-blood group ABO locus. We have determined the nucleotide sequence of this allele over the coding region in the last two coding exons. This allele does not possess the single-nucleotide deletion found common among all the O alleles previously analyzed. Compared with A^1 allele, this allele has three nucleotide substitutions resulting in two amino acid substitutions. The introduction of these amino acid substitutions into the A^1 transferase expression construct apparently abolished the enzymatic activity of A^1 transferase.
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McNeill et al. (1993) studied this question.