[Serological and Genotypic Analysis of a Family with B(A) Subtype]

To perform blood group typing and genotyping on the proband with weak ABO antigen expression and his family members, thereby clarifying the ABO phenotype and the molecular genetic basis of the weak antigen expression.

Serological ABO blood group typing was performed on the proband and four family members. PCR amplification of exons 6 and 7 of the ABO gene was carried out followed by direct sequencing analysis, and haplotypes were further confirmed by clonal sequencing.

Direct sequencing results revealed a c.640A＞G single-nucleotide heterozygous mutation in exon 7 of the proband's ABO gene. This mutation was confirmed by clonal sequencing and haplotype analysis to be located on the background of the B.01 allele, which was consistent with the characteristics of the ABO*BA.04 allele. Among the five members of the family, three carried the BA.04 allele. Specifically, the proband had a genotype of ABO*BA.04/O.01.01 with a serological phenotype of AwB; both the proband's son and eldest daughter had a genotype of ABO*BA.04/A1.01 with a serological phenotype of ABw.

When the ABO*BA.04 allele is heterozygous with other common alleles, different ABO weak antigen phenotypes can be produced. Serological testing combined with genotyping technology facilitates the accurate identification of ABO weak antigen phenotypes.