Background: The gene locus for the ABO blood group system encodes a glycosyltransferase. Alterations in the DNA sequence are associated with the blood groups and the expression levels of antigens on red blood cells. A number of ABO alleles have been described as the molecular basis of weak A or B antigens. Patients and Methods: Here, we describe a novel variant B allele in a blood donor with discrepant results in routine forward (group A) and reverse (very weak anti-B isoagglutinins) ABO blood grouping. Results: Determination of the ABO genotype using polymerase chain reaction-sequence-specific primers (PCR-SSP) indicated blood group A2B. Sequencing of the ABO gene exons 6 and 7 showed for 1 allele a G insertion into the GGGGGG sequence at position 811–816 of exon 7. The 816insG mutation (designated ABO*Bw20) led to a frame shift of the coding sequence and subsequent alteration of the protein sequence. The location of the mutation on a B allele was proven by PCR-SSP. Screening for the novel mutation in 211 blood donors with regular ABO phenotypes indicated that *Bw20 is a rare variant. Conclusions: The low levels of anti-B isoagglutinins associated with this novel variant indicate that residual undetectable amounts of B antigen may be present on red blood cells. The serological and molecular analysis of members of the blood donor’s family further proved the phenotype-genotype correlation of the *Bw20 allele with antigen·and individually variable levels of anti-B isoagglutinins. The characterization of novel alleles associated with ABO subgroups may ensure the correct determination of blood groups in which serological methods are combined with molecular genetic approaches.

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