As their name indicates, SSRIs, currently the most used AD, possess much higher selectivity for the 5-HT transporter (SERT) than for the transporters of norepinephrine (NET) and dopamine (DAT), and were developed after the serotonergic theory of depression postulated by Lapin and Oxenkrug (23) which suggests that serotonergic pathways play a major role in depression. Here, SLC6A4 is linked to major depressive disorder.