UGT3A1 is a UDP-glycosyltransferase that catalyzes phase II biotransformation through conjugation of lipophilic substrates to enhance water solubility and excretion. Unlike the predominant UGT1 and UGT2 families that use UDP-glucuronic acid, UGT3A1 uniquely utilizes UDP N-acetylglucosamine as its preferred sugar donor 1, a primate-specific adaptation conferred by asparagine-391 in its active site 2. Primary substrates include ursodeoxycholic acid, bile acids, estrogens (17α- and 17β-estradiol), and xenobiotics such as 4-nitrophenol 1. The enzyme is predominantly expressed in liver and kidney, with lower gastrointestinal expression 1, showing age-dependent upregulation with maximal expression in the elderly 3. Clinically, UGT3A1 plays important roles in ursodeoxycholic acid metabolism during cholestasis therapy and gallstone dissolution 1. A polymorphic C121G variant results in complete catalytic inactivity 1. Emerging evidence suggests UGT3A1 participates in polycyclic aromatic hydrocarbon metabolism, though with limited activity compared to UGT3A2 4. GWAS studies identified UGT3A1 as a miRNA-targeted gene associated with primary biliary cirrhosis susceptibility 5. UGT3A1 expression correlates with UGT3A2 across all populations but independently of other UGT isoforms in sex- and age-dependent patterns 3.