One study of the monoamine oxidase gene, MAOA, which has been implicated in multiple mental disorders, examined mRNA expression imbalances between two single nucleotide polymorphisms (SNPs), or sites of known DNA variation, measured methylation in the gene promoter, and genotyped subjects for a repeat polymorphism in the promoter (Pinsonneault et al. 2006). Here, MAOA is linked to mental disorder.