The cis-AB is a very rare phenotype in the ABO blood group system. It corresponds to a special ABO allele encoding a glycosyltransferase that is capable of synthesizing both A and B antigens. Until now, gene sequences of only 3 cis-AB alleles were characterized. One was the A(1v) allele with a nucleotide substitution G803C at codon 268; the second was the B allele with a nucleotide substitution A796C at codon 266; and the third arose from a point mutation C700T at codon 234 in exon 7 of the B transferase gene. In this study, we found a novel cis-AB allele when performing paternity tests in Chang Gung Memorial Hospital in Taiwan. Although his father was O blood type, a serologically AB blood type child was confirmed as being his father's offspring on the basis of 16 microsatellite markers (99.97% plausibility for the child and father). Exons 6 and 7 of the child's ABO alleles were characterized by direct sequencing and gene cloning. The results showed that the child has one O(1) allele and the second allele is almost identical to A(1*02) allele except for a single point mutation at nucleotide position 796, where an A replaces a C and leads to a change of leucine to methionine at amino acid 266. This implies that the child's O(1) allele was inherited from his father and the other allele was inherited from his mother. In conclusion, the novel cis-AB allele reported here is derived from the A transferase gene through a nucleotide substitution C796A, which differs from the 3 previously reported cis-AB alleles.