The function of UGT1A9 (UDP glucuronosyltransferase family 1 member A9, Ensembl gene identifier ENSG00000241119) is as follows. [Isoform 1]: UDP-glucuronosyltransferase (UGT) that catalyzes phase II biotransformation reactions in which lipophilic substrates are conjugated with glucuronic acid to increase the metabolite's water solubility, thereby facilitating excretion into either the urine or bile (PubMed:12181437, PubMed:15470161, PubMed:15472229, PubMed:18004212, PubMed:18052087, PubMed:18674515, PubMed:19545173, PubMed:15231852, PubMed:21422672, PubMed:38211441). Essential for the elimination and detoxification of drugs, xenobiotics and endogenous compounds (PubMed:12181437, PubMed:18004212). Catalyzes the glucuronidation of endogenous estrogen hormones such as estradiol and estrone (PubMed:15472229). Involved in the glucuronidation of arachidonic acid (AA) and AA-derived eicosanoids including 15-HETE, PGB1 and F2-isoprostanes (8-iso-PGF2alpha and 5-epi-5-F2t-IsoP) (PubMed:15231852, PubMed:38211441). Glucuronates the phytochemical ferulic acid efficiently at both the phenolic or the carboxylic acid group (PubMed:21422672). Also catalyzes the glucuronidation of the isoflavones genistein, daidzein, glycitein, formononetin, biochanin A and prunetin, which are phytoestrogens with anticancer and cardiovascular properties (PubMed:18052087, PubMed:19545173). Involved in the glucuronidation of the AGTR1 angiotensin receptor antagonist caderastan, a drug which can inhibit the effect of angiotensin II (PubMed:18674515). Involved in the biotransformation of 7-ethyl-10- hydroxycamptothecin (SN-38), the pharmacologically active metabolite of the anticancer drug irinotecan (PubMed:12181437, PubMed:20610558). Also metabolizes mycophenolate, an immunosuppressive agent (PubMed:15470161, PubMed:18004212). {ECO:0000269|PubMed:12181437, ECO:0000269|PubMed:15231852, ECO:0000269|PubMed:15470161, ECO:0000269|PubMed:15472229, ECO:0000269|PubMed:18004212, ECO:0000269|PubMed:18052087, ECO:0000269|PubMed:18674515, ECO:0000269|PubMed:19545173, ECO:0000269|PubMed:20610558, ECO:0000269|PubMed:21422672, ECO:0000269|PubMed:38211441}. [Isoform 2]: Lacks UGT glucuronidation activity but acts as a negative regulator of isoform 1. {ECO:0000269|PubMed:18004212, ECO:0000269|PubMed:20610558}.