SLC6A4 and major depressive disorder: In this study we found no statistically significant associations between peripheral DNA methylation of two key regulatory genes of serotonin neurotransmission (SLC6A4 and TPH2) and brain levels of 5-HTT and 5-HT4 in a cohort of healthy participants or 5-HT4 in a cohort of unmedicated patients with MDD.