The area of the B transferase surrounding codons 266 and 268 is schematically shown. With the original alanine (A) at codon 268, a galactose fits well to the cavity space. When the alanine is replaced by the glycine (G) residue, the space becomes bigger and either a GalNAc or a galactose fits within the space. When the alanine is replaced by bigger amino acids, the efficiency becomes lower depending on their sizes. The charge of the amino acid at codon 268 is also important.