Two human liver UDP-glucuronosyltransferase (transferase) cDNAs, HUG-Br1 and HUG-Br2, were previously isolated (Ritter, J. K., Crawford, J. M., and Owens, I. S. (1991) J. Biol. Chem. 266, 1043-1047), and each was shown to encode a bilirubin transferase isozyme which catalyzes the formation of all physiological conjugates of bilirubin IX alpha following expression in COS-1 cells. Sequence data showed that the cDNAs contained identical 3' ends (1469 base pairs in length) to each other and to that of the human phenol transferase cDNA, HLUG P1 (Harding, D., Fournel-Gigleux, S., Jackson, M. R., and Burchell, B. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 8381-8385). Here we report that the two corresponding bilirubin transferases and the phenol transferase are encoded by a novel locus, UGT1, which is also predicted to encode three other bilirubin transferase-like isozymes all having identical carboxyl termini. The transcriptional arrangement utilizes six nested promoter elements, each of which is positioned upstream of a unique exon 1. Each exon 1 encodes the NH2-terminal domain (286 amino acids) and confers the substrate specificity of the isoform. The 3' end of the locus contains 4 common exons which encode the identical carboxyl termini (246 amino acids). It is predicted that six nested primary transcripts are synthesized and that each exon 1 is differentially spliced to the 4 common exons to produce six unique, mature mRNAs. Although the gene organization is present as a single copy, it provides the flexibility of independent regulation of each isoform which is known to occur in the case of bilirubin and phenol transferase activities. With an understanding of the gene structure, lethal, as well as the nonlethal defects, associated with bilirubin transferase activity can now be determined.