We constructed a cDNA expression construct of the mouse ABO gene and determined specificity. We also constructed the Human-mouse chimeric construct by replacing the last coding exon of the human construct with the mouse counterpart. Four constructs were used as controls: no DNA, pA(arg), pAAAA, and pBBBB. The pA(arg) construct encodes A transferase, except that codon 268 was replaced from glycine to arginine. Due to the amino acid substitution, this construct cannot exhibit A transferase activity, and therefore was used as a negative control. pAAAA and pBBBB constructs encode functional A and B transferases, respectively. Therefore, they were used for positive controls. These constructs were transfected to the HeLa cell, the human uterine cancer cells, and the appearance of A and B antigens were monitored by the immunological method using anti-A and anti-B antibodies. Both the pHuman-mouse and pMouse constructs induced the expression of A and B antigens, suggesting that the mouse gene is a cis-AB gene and encodes an enzyme with both A and B transferase activity.