The area of the A transferase surrounding codons 266 and 268 is schematically shown. With the original glycine (G) residue at codon 268, only a GalNAc fits well to the cavity space. When it is replaced by the alanine (A) residue, the space becomes a little smaller. As a result, not only a GalNAc but also a galactose are able to fit in this space. The situation is the same for the replacement of the glycine residue with serine (S) or cysteine (C), but when it is replaced by a larger residue, neither a GalNAc nor a galactose fits in the space any longer.