The blood group ABO gene codes for a glycosyltransferase that adds the ultimate monosaccharide to a glycoconjugate and forms the A or B blood group specific antigen. The DNA structure of the three major alleles of the human blood group ABO system was first described in 1990. This review describes the subsequent developments, including the increasing number of variants of these common alleles and the underlying mutations thought to be responsible for the occurrence of some of the weak subgroups of blood group A and B. Several inactive (O) alleles are also now known. Our knowledge of the DNA sequence of the normal A and B alleles and of the rare and intriguing cisAB and B(A) phenotypes has resulted in plausible explanations for these. Allelic variations outside the translated exons have been investigated and resulted in detection of lineage-specific intron mutations and the discovery of an enhancer VNTR region affecting the rate of transcription at this locus. The occurrence of hybrid alleles can also explain hitherto abnormal inheritance in some pedigrees. The detection of hybrid alleles has been made possible by the presence of numerous polymorphisms found in the various ABO alleles. The role of chi (chi) sequences is discussed. Finally, the various genotyping methods available are summarized and their advantages and limitations are analysed in the light of the increasing allelic variation.