The pathophysiology of depression and epilepsy might result, at least in part, directly from a dysregulation of brain serotonin 2A neurotransmission or indirectly from the dysfunction of other neurotransmitter systems (i.e., dopaminergic, glutamatergic, GABAergic) that are under 5-HT2A control. This evidence concerns the gene HTR2A and depressive disorder.