The article reviews the progress in understanding the structure, regulation, and function of monoamine oxidase (MAO) A and B, focusing on their roles in neurotransmitter metabolism and behavior. MAO A and B are located on the X-chromosome and consist of 15 exons with identical intron-exon organization, suggesting they derive from a common ancestral gene. Studies using knockout (KO) mice have revealed distinct differences in neurotransmitter metabolism and behavior between MAO A and B. MAO A knockout mice exhibit elevated brain levels of serotonin, norepinephrine, and dopamine, and show aggressive behavior similar to human males with a deletion of MAO A. In contrast, MAO B knockout mice do not exhibit aggression but have increased levels of phenylethylamine. Both KO mice show increased reactivity to stress. The article also discusses the involvement of MAO in Parkinson's disease, smoking, alcoholism, and stress-related disorders. The findings highlight the importance of MAO in various physiological and pathological processes and suggest potential therapeutic targets for neurodegenerative and psychiatric disorders.The article reviews the progress in understanding the structure, regulation, and function of monoamine oxidase (MAO) A and B, focusing on their roles in neurotransmitter metabolism and behavior. MAO A and B are located on the X-chromosome and consist of 15 exons with identical intron-exon organization, suggesting they derive from a common ancestral gene. Studies using knockout (KO) mice have revealed distinct differences in neurotransmitter metabolism and behavior between MAO A and B. MAO A knockout mice exhibit elevated brain levels of serotonin, norepinephrine, and dopamine, and show aggressive behavior similar to human males with a deletion of MAO A. In contrast, MAO B knockout mice do not exhibit aggression but have increased levels of phenylethylamine. Both KO mice show increased reactivity to stress. The article also discusses the involvement of MAO in Parkinson's disease, smoking, alcoholism, and stress-related disorders. The findings highlight the importance of MAO in various physiological and pathological processes and suggest potential therapeutic targets for neurodegenerative and psychiatric disorders.