UGT2B4 is a UDP-glucuronosyltransferase that catalyzes phase II biotransformation reactions, conjugating lipophilic substrates with glucuronic acid to enhance water solubility and facilitate excretion 1. This enzyme plays a crucial role in metabolizing endogenous steroid hormones, bile acids, and xenobiotics including drugs like carvedilol 1. UGT2B4 exhibits broad tissue expression in liver, kidney, prostate, breast, and other extrahepatic tissues 2. The enzyme demonstrates substrate specificity dependent on critical amino acid residues, particularly an aromatic residue at position 33 that is essential for activity toward substrates like hyodeoxycholic acid and catechol-estrogens 3. UGT2B4 shows significant genetic polymorphism with functional variants like D458E, displaying different allele frequencies across ethnic populations 45. Notably, the upstream regulatory region of UGT2B4 shows evidence of balancing selection, maintaining two major haplotypes that may optimize enzyme expression levels 6. This regulatory variation has been associated with breast cancer risk, likely due to UGT2B4's role in steroid hormone metabolism 6. The gene can be induced by farnesoid X receptor ligands and shows enhanced expression in 3D cell culture models 7.