UGT1A3 is a UDP-glucuronosyltransferase that catalyzes glucuronidation of diverse endogenous and exogenous compounds in human liver and colon tissue 1. The enzyme metabolizes estrogens (estrone and 2-hydroxyestrone) 1, flavonoids including quercetin, luteolin, and kaempferol 2, vitamin D metabolites (25-hydroxyvitamin D3) 3, and sartans with high regioselectivity toward the N2 position of tetrazole rings 4. UGT1A3 also contributes to glucuronidation of the natural flavonoid icaritin, accounting for 37.5% of one glucuronide formation in human liver 5. The enzyme exhibits significant interindividual variability in activity. Six known single nucleotide polymorphisms alter enzyme function; variants UGT1A3.2, UGT1A3.3, and UGT1A3.5 show markedly reduced activity, while UGT1A3.4 demonstrates approximately 4-fold increased glucuronidation efficiency toward quercetin 2. Allele frequencies differ between populations 2. UGT1A3 expression is regulated by transcription factors CAR, PXR, and ESR1 6, and can be induced by pregnane X receptor agonists like rifampin 3. Variants may alter human susceptibility to flavonoid exposure and vitamin D homeostasis, with potential clinical relevance for drug-drug interactions and xenobiotic metabolism.