FMO5 (flavin-containing dimethylaniline monooxygenase 5) is a hepatic monooxygenase localized to chromosome 1 1 that catalyzes Baeyer-Villiger oxidation, inserting oxygen into carbon-carbon bonds adjacent to carbonyls to convert ketones to esters 234. Unlike other FMO family members, FMO5 exhibits poor activity on classical xenobiotic substrates (drugs, pesticides, dietary components) but effectively oxidizes diverse carbonyl compounds, particularly drug molecules with aliphatic carbonyl chains such as nabumetone and pentoxifylline 54. FMO5 also demonstrates NADPH oxidase activity in substrate-free conditions 3. Beyond xenobiotic metabolism, FMO5 regulates glucose homeostasis and cholesterol biosynthesis through co-expression with PPARα 6. In alcoholic fatty liver disease, reduced FMO5-PPARα co-expression activates pro-inflammatory NF-κB signaling, promoting hepatocyte apoptosis and injury 6. In idiopathic pulmonary fibrosis, FMO5 is downregulated as a cuproptosis-related gene 7. Human FMO5 variants correlate with altered metabolic phenotypes including lower insulin levels and reduced waist circumference 8, suggesting FMO5 influences metabolic aging and metabolic disease risk.