the abo blood group is the most important blood group system in transfusion medicine. antigens of the abo system consist of a or b carbohydrate structure carried on the substrate h antigen the abo gene is responsible for encoding for glycosyltransferases a or b that defines which specific carbohydrate is added to the end of h substance oligosaccharide chains, galnaca1-3 and gala1-3, respectively. the dna structure of the three major alleles of the human blood group abo system, a1 and b, was first described in 1990. advances of molecular genetics have allowed understanding of the molecular basis of the abo blood group system and the knowledge of the common alleles polymorphisms of this locus. this review article has the purpose of describing the variants of these alleles and the underlying mutations, deletions or rearrangement of the genes responsible for the occurrence of abo subgroups and o transferases inactivation. finally, various methods available for abo genotyping are also evaluated, as well as its advantages and limitations.