The domestic cat has a significantly lower capacity to glucuronidate planar phenolic xenobiotics compared with most other mammalian species. The aim of this study was to determine the mechanistic basis for this anomaly. Current knowledge of the substrate specificity of UDP-glucuronosyltransferase (UGT) isoforms indicates that the cat may either lack or poorly express UGT1A6. Initially, a novel cloning technique was used to identify UGT1A genes expressed in cat liver. Only two unique UGT1A isoforms could be discriminated. The first (28%, of clones) was most homologous to UGT1A1 (the bilirubin-UGT), while the second (72% of clones) showed homology to several isoforms, but could not be unambiguously identified, and was designated cat UGT1A02. Southern blot analysis confirmed the presence of a single UGT1A6-homologous region in the cat genome. Subsequent cloning and sequencing of the entire UGT1A6 exon 1 coding region revealed five deleterious genetic mutations. Identical mutations were found by sequencing of UGT1A6 exon 1 from five other unrelated cats. Four of these five genetic lesions were also identified in the UGT1A6 exon 1 region of a margay (Leopardus wiedii). Finally, RT-PCR of liver mRNA from four different cats confirmed the presence of UGT1A1 and UGT1A02, but not UGT1A6. In conclusion, UGT1A6 is a pseudogene in the domestic cat and in at least one other phylogenetically related species. Furthermore, cats appear to have a less diverse pattern of UGT1A isoform expression compared with other species. Such differences most likely reflect the highly carnivorous diet of Feliform species and resultant minimal exposure to phytoalexins.