The MAOA monoamine-oxidase A gene, encodes an enzyme partially responsible for the metabolism of several neurotransmitters such as dopamine and serotonin. The monoamine oxidase family of enzymes, which metabolize monoamines (neurotransmitters and neuromodulators consisting of a single amine), includes MAOA and MAOB.
Although not a SNP per se, the variation that has been most studied consists of a 30 base-pair variable number tandem repeat (VNTR) located in the promoter region of the gene. Alleles with 3.5 and 4 repeats are 2-10 times more productive than the allele with 3 repeats. Several studies have shown an association between the 3-repeat allele and neuropsychiatric conditions such as alcoholism, antisocial personality, impulsivity, and poor reaction to stress, leading some popular media to label this allele the "warrior gene".
Brunner syndrome: a recessive X-linked disorder characterized by impulsive aggressiveness and mild mental retardation resulting from MAOA deficiency OMIM; dbSNP reports a probably pathogenic allele detected by SNP rs72554632 C/T CAT->TAG QLN->STOP(AMBER), the same SNP associated with Brunner syndrome.