In addition to the weak subgroup alleles, we also characterized mutations in the alleles that specified interesting phenomena of cis-AB and B(A). Cis-AB is a phenomenon where the expression of both A and B antigens are specified by a single allele, as opposed to regular trans-AB, which is the combination of A and B alleles from the father and mother. The cis-AB allele usually specifies a weak expression of A and a weaker expression of B (phenotypically A2B3) (Yamaguchi et al., 1965, 1966). B(A) was discovered when certain monoclonal anti-A antibodies were found to react with RBCs that were previously typed as B (Treacy and Stroup, 1987). B(A) is similar to cis-AB in the sense that one allele specifies the expression of both A and B antigens, however, B(A) specifies weak A antigen expression in addition to strong B antigen expression. By PCR amplification of genomic DNA and following DNA sequencing, we identified mutations in the cis-AB01 and B(A)01 alleles (Yamamoto et al., 1993b; Yamamoto et al., 1993c). We found that the proteins encoded by these alleles are A-B transferase chimeras. The cis-AB01 allele had a coding sequence identical to the A101 allele, except for G268A (the last of the four amino acid substitutions that discriminate the A and B transferase is alanine of B transferase). Conversely, the B(A)01 allele had a coding sequence identical to the B101 allele, except that the second of 4 amino acid substitutions is the amino acid of A transferase (G at codon 235 in place of S).