The ABO blood group is the most important blood group system in transfusion medicine. Since the ABO gene was cloned and the molecular basis of the three major alleles delineated about 10 years ago, the gene has increasingly been examined by a variety of DNA-based genotyping methods and analysed in detail by DNA sequencing. A few coherent observations emerge from these studies. First, there is extensive sequence heterogeneity underlying the major ABO alleles that produce normal blood groups A, B, AB and O when in correct combination with other alleles. Second, there is also extensive heterogeneity underlying the molecular basis of various alleles producing ABO subgroups such as A # , A x and B $ . There are over 70 ABO alleles reported to date and these alleles highlight the extensive sequence variation in the coding region of the gene. A unifying system of nomenclature is proposed to name these alleles. Third, extensive sequence variation is also found in the non-coding region of the gene, including variation in minisatellite repeats in the 5h untranslated region (UTR), 21 single nucleotide polymorphisms (SNPs) in intron 6 and one SNP in the 3h UTR. The haplotypes of these variations reveal a specific relationship with the major ABO alleles. Fourth, excluding the common alleles, about half of the remaining alleles are due to new mutations and the other half can better be explained by intragenic recombination (both crossover and gene conversion) between common alleles. In particular, the recombination sites in hybrid alleles can be quite precisely defined through haplotype analysis of the SNPs in intron 6. This indicates that recombination is equally as important as point mutations in generating the genetic diversity of the ABO locus. Finally, a large number of ABO genotyping methods are available and are based on restriction analysis, allele specific amplification, mutation screening techniques or their combinations.
The ABO blood group system (MIM number : 110300) was discovered by K Landsteiner a century ago. It is the most important blood group system in transfusion medicine. The serological and genetic characteristics of the system and the biosynthesis of its antigens have been well established (Daniels, 1995 ;Watkins, 1980 The antigenic determinants of the system are oligosaccharides found on glycoproteins and glycolipids. As such they are not direct gene products of the ABO gene. Instead, the ABO gene encodes enzymes known as glycosyltransferases which transfer specific sugar residues to a precursor substance (the H antigen) to produce the A and B antigens. There are three major alleles at the ABO locus : alleles A, B and O. The A allele encodes α1 4 3 N-acetylgalactosaminyltransferase (A transferase ; EC 2n4.1n40) which adds N-acetylgalactosamine (GalNAc) to the H antigen to form the A antigen. The B allele encodes α1 4 3 galactosyltransferase (B transferase ; EC 2n4.1n37) which transfers galactose
