BACKGROUND: Today we know more than 200 ABO variant alleles. Most ABO subtypes are caused by single nucleotide substitutions in exons 6 or 7, which encode for the catalytic domain of ABO glycosyltransferases. Understanding the molecular basis of these variants provides insights into the mechanisms responsible for the different phenotypes. We investigated the molecular background causing inconclusive ABO serology of five samples referred to our laboratory. METHODS: The ABO phenotypes were determined by standard gel column agglutination and tube technique as well as flow cytometry. Molecular investigation included sequence-specific primer (SSP)-PCR and sequencing of all seven ABO exons containing the adjacent flanking intron regions (including the ABO regulatory regions) using published and in-house primers. RESULTS: Serological and molecular analysis of five probands revealed diverse ABO blood group variations. Four probands showed weak A antigen expression and one proband weak B antigen expression. Proband 1 exhibited a c.952G>
A (p.Val318Met) variant linked to the ABO*A1.01 background, while proband 2 carried a c.973T>
C (p.Trp325Arg) and proband 3 a c.28+5G>
A splice site variant associated with the same allele. Proband 4 revealed the c.407C>
A (p.Thr136Lys) variant on an ABO*A2.01 background, and proband 5 had the c.860C>
T (p.Ala287Val) variant associated with the ABO*B.01 background. DISCUSSION: Here we show the serological and molecular analysis of five samples with serologic ABO blood grouping attenuations and/or discrepancies revealing novel A and B subgroup alleles. We report five novel variant alleles: ABO*A1.952A, ABO*A1.973C, ABO*A1.28+5A, ABO*A2.01.407A, and ABO*B1.860T.