SLC6A4 and major depressive disorder: The neurotransmitter serotonin (5-HT) is known to play important roles in synaptic plasticity and mood/emotion, and the level of DNA methylation in the promoter of the SLC6A4 gene, which encodes serotonin transporter (SERT), has been shown to be associated with its mRNA expression as well as depression and antidepressant responsiveness (Philibert et al., 2007; Sugawara et al., 2011; Okada et al., 2014).