The function of ABO (ABO, alpha 1-3-N-acetylgalactosaminyltransferase and alpha 1-3-galactosyltransferase, Ensembl gene identifier ENSG00000175164) is as follows. 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 N-acetyl-alpha- D-galactosamine (GalNAc)) or to the B antigen (by addition of galactose (Gal)), whereas O individuals lack such activity and express the H antigen precursor unmodified. Catalyzes the transfer of GalNAc or Gal in an alpha1,3 linkage to the terminal Gal of all five types of naturally occurring H antigens, forming the antigenic structures of the A and B blood groups. {ECO:0000269|PubMed:12198488, ECO:0000269|PubMed:12529355, ECO:0000269|PubMed:16326711, ECO:0000269|PubMed:17259183, ECO:0000269|PubMed:2121736, ECO:0000269|Ref.20}.