SLC6A4 and major depressive disorder: The serotonin transporter-linked polymorphic region (5-HTTLPR) within the promoter region of SLC6A4 has been implicated in depression (Clarke et al., 2010; Wang et al., 2016), anxiety-related traits (Munafò et al., 2009b), and PTSD (Xie et al. 2009).