PER2 and major depressive disorder: Nevertheless, several genes have been proposed to be involved in the etiology of both depression and AUD, exemplified by brain-derived neurotrophic factor (BDNF), neuropeptide Y (NPY), dopamine receptor D2 (DRD2), catechol-O-methyltransferase (COMT), monoamine oxidase A (MAOA), period homolog 2 (PER2) and several subtypes of the serotonin receptor [8-12].