We chose one of the A1 alleles that we identified as a standard and named this allele the A101 allele. The terminology was based on the phenotype plus a 2-digit number in the order of discovery, assuming that less than 100 alleles would be found for each individual allelotype. During the cloning of the ABO allelic cDNAs, we identified 2 kinds of O allelic cDNAs. One type has a single nucleotide deletion at nucleotide 261 when compared to A101, which was later named O01. Another type of O allelic cDNA contained several nucleotide substitutions in addition to the single nucleotide deletion. We later named this O allele O02. During the characterization of subgroup alleles, we also encountered another type of O allele that lacked the single nucleotide deletion. We found that this allele (O03) contained 2 amino acid substitutions (R176G and G268R), which seemed to abolish A transferase activity of the protein encoded by the allele (Yamamoto et al., 1993).