Genomic clones of UDP-glucuronosyltransferase family 1 (UGT1) were isolated from wild-type Wistar rats. The UGT1 locus spans > 120 kb and forms a gene complex. In this locus nine unique first exons encoding NH2-terminal portions of each isoform were located at intervals of approximately 10 kb and followed by only one set of commonly used exons (exons II, III, IV, and V) encoding the COOH-terminal portion. From sequence analyses of the unique first exons, the amino acid sequences of the isoforms were deduced and they were divided into two groups: the Bilirubin cluster (B cluster) and the Phenol cluster (A cluster). A and B clusters consisted of four (A1-A4) and five (B1-B5) isoform-specific exons, respectively. A2, A3, B3, and B4 were identified as previously uncharacterized forms, while A4 and B4 were pseudogenes. Isoform B1 was a major component in hepatic microsomes of untreated rats and was induced in clofibrate- and dexamethasone-administered rats. A slight but a significant amount of B1 mRNA was also detected in various tissues such as intestine. mRNAs coding for isoform A1 and isoform A2 were induced in livers of methylcholanthrene (MC)-treated rats. Induction of A1 mRNA was also observed in kidneys of MC-treated rats. A genomic clone containing the commonly used exons was also isolated from Gunn rats and a single base deletion was identified in exon IV. Isoforms of the UGT1 family are made from the complex gene locus by an alternative combination of one of the unique first exons with the commonly used exons.