摘要
Background: ABO blood system has many subgroups. In A group, A <sub>1</sub> phenotype and A <sub>2</sub> phenotype are more common, and A <sub>2</sub> is caused by deletion or substitution in A <sup>1</sup> allele (ABO*A1.01). Methods: Based on standard ABO serological test, the subject was identified as A <sub>2</sub> phenotype. Direct sequencing and ABO gene cloning were performed to analyze the allele. Results: The subject had one A <sup>1v</sup> allele (ABO*A1.02) and one O allele. The haplotype sequencing analysis of each allelic clone demonstrated that allele 1 was A <sup>1v</sup> (ABO*A1.02) allele with nt543 variation (543 G > C) and allele 2 was O <sup>1v</sup> allele (ABO*O.01.02) with nt261 deletion and nt220 variation. Conclusion: The 543 G > C nucleotide substitution of the present A <sup>1v</sup> allele (ABO*A1.02) shares the same sequence variation site with A <sup>x</sup> allele (ABO*AW.33) (543 G > T), and both 543 G > C and 543 G > T nucleotide substitutions encode the same amino acid change of tryptophan to cysteine. Mechanism, such as allelic enhancement, has been proposed to explain this controversial phenotype–genotype relationship. But in present case, there has been no B allele to enhance the expression of A <sub>x</sub> to that expected of A <sub>2</sub> , so there could be another novel underlying mechanism to be investigated.