A nomenclature system for the UDP glucuronosyltransferase superfamily is proposed, based on divergent evolution of the genes. A total of 26 distinct cDNAs in five mammalian species have been sequenced to date. Comparison of the deduced amino acid sequences leads to the definition of two families and a total of three subfamilies. For naming each gene, we propose that the root symbol UGT for human (Ugt for mouse), representing "UDP glucuronosyltransferase," be followed by an Arabic number denoting the family, a letter designating the subfamily, and an Arabic numeral representing the individual gene within the family or subfamily (hyphen before the Arabic number for mouse), e.g., human UGT2B1 and murine Ugt2b-1. Whereas the gene and cDNA should be italicized, the corresponding transcript, protein, and enzyme activity should not be written with lowercase letters or in italics, e.g., human or murine UGT2B1. Recent experimental evidence suggests that several exons of the UGT1 gene might be shared, indicating that distinct UGT1 transcripts and proteins may arise via alternative splicing; the gene and gene product of alternative splicing will be designated with an asterisk, e.g., UGT1*6 and UGT1*6, respectively. When an orthologous gene between species cannot be identified with certainty, as occurs in the UGT2B subfamily, we recommend sequential naming of the genes chronologically as they become characterized. We suggest that the human nomenclature system be used for species other than the mouse. We anticipate that this UGT gene nomenclature system will require updating on a regular basis.