FMO4 (flavin containing dimethylaniline monoxygenase 4) is a member of the flavin-containing monooxygenase family involved in xenobiotic metabolism and cellular protection mechanisms. The protein exhibits N,N-dimethylaniline monooxygenase activity and participates in the oxidative metabolism of various xenobiotics including drugs and pesticides 1. FMO4 demonstrates distinct tissue-specific localization patterns, with immunoreactivity detected in liver periportal regions (opposite to FMO1/FMO3 distribution) and kidney distal tubules, proximal tubules, collecting tubules, and glomerulus 2. Unlike some FMO family members, FMO4 protein expression has been confirmed in mouse, rat, and human liver and kidney microsomes 2. Related FMO family members exhibit additional protective functions beyond xenobiotic metabolism, including roles in lipid metabolism regulation (FMO2), cardioprotection through disulfide bond catalysis (FMO2), and angiogenesis promotion (FMO2) 345. The FMO family also contributes to TMAO synthesis, which has implications for metabolic dysfunction and reproductive health 67. While FMO4's specific physiological roles require further investigation, its unique tissue distribution and confirmed expression across species suggest important metabolic functions in detoxification and cellular homeostasis.