This protein is the basis of the ABO blood group system. The histo-blood group ABO involves three carbohydrate antigens: A, B, and H. A, B, and AB individuals express a glycosyltransferase activity that converts the H antigen to the A antigen (by addition of UDP-GalNAc) or to the B antigen (by addition of UDP-Gal), whereas O individuals lack such activity. an alpha-L-fucosyl-(1->2)-beta-D-galactosyl derivative + UDP-N-acetyl-alpha-D-galactosamine = an N-acetyl-alpha-D-galactosaminyl-(1->3)-[alpha-L-fucosyl-(1->2)]-beta-D-galactosyl derivative + H(+) + UDP an alpha-L-fucosyl-(1->2)-beta-D-galactosyl derivative + UDP-alpha-D-galactose = an alpha-D-galactosyl-(1->3)-[alpha-L-fucosyl-(1->2)]-beta-D-galactosyl derivative + H(+) + UDP Binds 1 Mn(2+) ion per subunit. Protein modification; protein glycosylation. Membrane-bound form in trans cisternae of Golgi. Secreted into the body fluid. The conserved DXD motif is involved in cofactor binding. The manganese ion interacts with the beta-phosphate group of UDP and may also have a role in catalysis. The soluble form derives from the membrane form by proteolytic processing. Genetic variations in ABO define the ABO blood group system [MIM:616093]. The ABO blood group system is the most important blood group system in blood transfusion. The sequence shown here is that of the A transferase. The B form differs by a few residues substitution. Residues 266 and 268 are important for specificity. The reference genome assembly (GRCh38/hg38) describes a non-functional O-type ABO allele. The O-type ABO allele results in a guanine deletion (NM_020469.2: c.286delG). This deletion induces a frameshift and creates a premature stop codon resulting in a truncated (117 amino acids) protein deprived of any glycosyltransferase activity (PubMed:2333095). Belongs to the glycosyltransferase 6 family. Histo-blood group ABO system transferase Histo-blood group ABO system transferase