Monoamine oxidases (MAOs) are enzymes located in the outer mitochondrial membrane, primarily found in nerve cells, liver cells, and other tissues. They catalyze the oxidative deamination of monoamine neurotransmitters, including dopamine, norepinephrine, and serotonin. MAOs are divided into two subtypes, MAO-A and MAO-B, which differ in gene expression and substrate specificity. MAO-A primarily acts on the metabolism of norepinephrine and serotonin, while MAO-B is mainly involved in the metabolism of phenylethylamine compounds. Mutations or abnormal expression of MAO genes are associated with various neurological and psychiatric disorders, including depression, anxiety disorders, and Parkinson's disease.