FMO1 (flavin-containing monooxygenase 1) is a broad-spectrum monooxygenase that catalyzes the oxygenation of nitrogen- and sulfur-containing compounds, including xenobiotics and endogenous metabolites 1. It catalyzes S-oxygenation of hypotaurine to produce taurine, which regulates cell volume and provides cytoprotective functions 1. FMO1 also catalyzes N-oxygenation of trimethylamine (TMA) to trimethylamine N-oxide (TMAO), potentially detoxifying this gut microbiota-derived metabolite 1. In human tissues, FMO1 shows marked developmental and tissue-specific expression: negligible in adult liver but highly expressed in kidney (47±9 pmol/mg) and fetal liver (14.4±3.5 pmol/mg), indicating it functions primarily as an extrahepatic drug-metabolizing enzyme in adults 2. Within skin, FMO1 marks a major fibroblast population (larger cells in interstitial and perivascular locations) distinct from SFRP2+ fibroblasts, suggesting roles in tissue structure and homeostasis 3. Disease relevance includes TMAO-mediated metabolic dysfunction, as TMAO activates the ER stress kinase PERK, driving metabolic syndrome progression 4. FMO1 metabolizes pesticides including phorate and fenthion to their sulfoxide metabolites 5, indicating its clinical significance in xenobiotic detoxification and drug metabolism across renal and intestinal tissues.