MAOA and major depressive disorder: Related candidate genes can regulate the level of neurotransmitters (serotonin or dopamine) in the synaptic space through degradation (e.g., MAOA, COMT) and transport (such as 5-HTTLPR), and can also change the number of receptors in the brain (5HT2A, DRD2 gene) to regulate signal transmission, which in turn affects the level of individual depression.