Drug-metabolizing enzymes (phase Ⅰ,Ⅱ,and Ⅲ) play important roles in drug interaction,efficacy,and clearance.UDP glucuronosyltransferase proteins (UGTs) are a large family of phase Ⅱ drug-metabolizing enzymes that conjugate numerous pharmaceutical drugs,facilitating their excretion from our body.These diverse enzymes are encoded by clusters of UGT genes in vertebrates.Vertebrate UGT genes have a striking variable and constant genomic organization,similar to that of the immunoglobulin,T-cell receptor,and neural protocadherin gene clusters.Specifically,multiple highly-similar exons are arranged in a tandem array.This array of multiple variable exons is followed by a single set of downstream constant exons.Each variable exon is preceded by its own promoter and is separately spliced to the common set of constant exons to generate diverse UGT mRNA molecules.Thus,a combination of differential promoter activation and alternative splicing determines tissue-specific expression patterns of UGT genes.