So we see that the antigens in the ABL system are all glycosylated glycoproteins. And what that means is not only do we have the protein on the antigen, we also have this oligosaccharide as shown in this diagram. Now it turns out that the terminal sugar residues on the antigen determines the type of blood group as we discussed just a moment ago. So if we have this type of linkage and this type of additional sugar, that's the A. If we have this one, that's the B. If we have neither, then it's the O.