ABO, also known as NAGAT, belongs to the glycosyltransferase 6 family. This protein is the basis of the ABO blood group system and related to the first discovered blood group system, ABO. Which allele is present in an individual determines the blood group. 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. Recombinant human ABO protein, fused to His-tag at N-terminus, was expressed in E. coli and purified by using conventional chromatography techniques.