UGT1A9 is a UDP-glucuronosyltransferase enzyme primarily involved in Phase II metabolism of xenobiotics and endobiotics through glucuronidation. 1 UGT1A9 serves as a primary enzyme for glucuronidating isoflavones including tectorigenin and irigenin, alongside UGT1A1. 2 The enzyme metabolizes clinically important substrates such as propofol and mycophenolic acid (MPA), making it relevant for anesthetic and immunosuppressive drug disposition. 3 UGT1A9 gene polymorphisms, particularly at the -440C/T locus, correlate with propofol efficacy during anesthesia, with CC genotype carriers showing faster drug clearance. 4 A promoter polymorphism (UGT1A9*22) containing an insertion in the poly-T region increases transcriptional activity 2.6-fold, with allele frequencies varying by ethnicity (60% Japanese, 39% Caucasian, 44% African-American). 5 Sleep deprivation decreases hepatic UGT1A9 expression through disruption of circadian clock regulation by BMAL1, prolonging propofol anesthesia effects. 6 In kidney transplant recipients receiving MPA, UGT1A9 polymorphisms (particularly 275T>A) associate with rejection risk, with TT genotype showing decreased rejection. 7 UGT1A9 variants have minimal clinically meaningful effects on ertugliflozin exposure (within ±10% of wild-type), not requiring dose adjustment.