The function of Ensembl gene identifier ENSG00000244474 (UGT1A4, UDP glucuronosyltransferase family 1 member A4) 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:18177842, PubMed:24641623, PubMed:15231852). Essential for the elimination and detoxification of drugs, xenobiotics and endogenous compounds (PubMed:18177842, PubMed:20133892). Involved in the glucuronidation of calcidiol, which is the major circulating form of vitamin D3 essential for the regulation of calcium and phosphate homeostasis (PubMed:24641623). Also glucuronidates the biologically active form of vitamin D3, calcitriol, probably leading to its biliary transport and intestinal reabsorption (PubMed:18177842). Involved in the glucuronidation of arachidonic acid (AA) and AA-derived eicosanoids including 15-HETE, 20-HETE and PGB1 (PubMed:15231852). Catalyzes N- glucuronidation of tobacco alkaloids, tobacco-specific nitrosamines and tricyclic antidepressants (PubMed:17576790, PubMed:18238858, PubMed:20133892, PubMed:28803208). {ECO:0000269|PubMed:15231852, ECO:0000269|PubMed:17576790, ECO:0000269|PubMed:18177842, ECO:0000269|PubMed:18238858, ECO:0000269|PubMed:20133892, ECO:0000269|PubMed:24641623, ECO:0000269|PubMed:28803208}. [Isoform 2]: Lacks UDP-glucuronosyltransferase (UGT) activity but acts as a negative regulator of isoform 1. .