SLC6A4 and major depressive disorder: Genetic polymorphisms in serotonin genes [e.g., 5-HT transporter (5-HTT), 5-HT1A receptor] have been associated with depression, but these associations are weak and not always reproducible, suggesting that serotonin may be a predisposing factor rather than a cause of depression or anxiety (Karg et al., 2011; Kishi et al., 2013).