The deduced amino acid sequences surrounding codons 266 and 268 of the human A and B transferases and corresponding sequences of evolutionarily-related genes are shown. The asterisks indicate the positions of codons 266 and 268. Human A transferase has leucine (L) and glycine (G) at those positions, whereas human B transferase has methionine (M) and alanine (A). The human O03 allele has leucine and arginine (R), and because of the arginine, the protein has no enzymatic activity. The cis-AB01 allele is an A-B transferase chimera and has leucine of A transferase and alanine of B transferase. The mouse AB gene has alanine at codon 268 as in human B transferase, but contains glycine, which is much smaller than methionine of human B transferase. This allows the transfer of both a GalNAc and a galactose in the in vitro transfection experiments using human HeLa cells as the host. However, A antigens are primarily synthesized in vivo. Pig A gene has alanine and glycine at codons 266 and 268. Because the glycine at codon 268 is the same as in human A transferase and the alanine at codon 266 is smaller than the leucine in human A transferase, the enzyme transfers only a GalNAc. Dog Forssman glycolipid synthase has glycine and alanine, the same two amino acids as the mouse cis-AB gene. However, possibly because valine at codon 269 is smaller than phenylalanine (F), the protein transfers only a GalNAc. Both murine and bovine α1-3 Gal transferases have histidine (H) and alanine. Because alanine is the same as alanine in the human B transferase and histidine is bigger than methionine, they transfer only a galactose.