Once nucleotide sequences were determined for the three major alleles of A (A1), B, and O, we deduced the amino acid sequences of their coding regions. We found that O alleles encode truncated proteins due to the single nucleotide deletion (261delG) described above. This explained the non-functionality of O alleles. We also found that there are several nucleotide substitutions between A and B alleles - 4 of which result in amino acid substitutions. Those are arginine (R), glycine (G), leucine (L), and glycine (G) at codons 176, 235, 266, and 268 in A transferase, and glycine (G), serine (S), methionine (M), and alanine (A) at the same codons in B transferase.