We have determined the nucleotide sequence of the coding region in the last two coding exons of ABO genes from two c/k-AB individuals (genotype cis- AB/O) with no consanguinity. In this region, ris-AB alleles from these 2 individuals were identical to one another while different from the A^1 allele by two nucleotide substitutions. Both of these nucleotide substitutions result in amino acid substitutions. The first substitution is identical to the one previously found in the A^2 allele. The other substitution is found at the fourth position of the four amino acid substitutions which discriminate A^1 and B transferases.
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McNeill et al. (1993) studied this question.