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. Here, SLC6A4 is linked to major depressive disorder.