The MAO-A gene or so-called "warrior gene" encodes the monoamine oxidase A (MAO-A) enzyme in the brain that breaks down neurotransmitters such as noradrenaline, adrenaline, serotonin, and dopamine. Signals transmitted by serotonin regulate mood, emotion, sleep, and appetite. Epinephrine and norepinephrine control the body's response to stress. Dopamine transmits signals within the brain to produce smooth physical movements.
Monoamine oxidase A also helps break down monoamines found in the diet. It seems to be particularly important in the breakdown of excess tyramine, which is found in cheese and other foods.
If you have high levels of this enzyme, it means you will have fewer neurotransmitters. If you have low levels of this enzyme, you'll have more neurotransmitters. MAO-A is not something you want to be too high or too low because in both situations it will cause different negative effects.
If you carry the T-allele in this gene then you may be predisposed to an increased risk of major depressive disorder and bipolar disorder.
If you carry the C-allele then you may be predisposed to a decrease in spatial memory and reduced response to antidepressant drugs.
MAO-A uses vitamin B2 and copper as cofactors in order to function correctly.