In addition to the analysis of primate ABO genes, we also cloned cDNAs from porcine and murine ABO genes (Yamamoto and Yamamoto, 2001; Yamamoto et al., 2001). We demonstrated that the murine gene is well-conserved within the species and encodes an enzyme with both A and B transferase activity in the in vitro assay (although A antigens are primarily synthesized in vivo). Pigs (porcine genes) exhibit AO polymorphism. Our study showed that the most, if not all of the structural gene is missing in type O pigs, and therefore, inadvertent expression of an A antigen in xenotransplanted tissues/organs from O pigs will be unlikely. Based on our studies, we constructed an evolutionary tree of ABO and related genes. Those related genes included α1-3 Gal transferase genes that synthesize α1-3 Gal epitope from mouse, pig, and cow samples, and a Forssman synthase gene that had been cloned from dog by that time. The murine and porcine ABO genes clustered with the human ABO genes, rather than with the α1-3 Gal transferase genes and the Forssman synthase gene.