SLC6A4 and major depressive disorder: In humans, a repeat length polymorphism (5HTTLPR) in the transcriptional control region of the 5-HTT gene (SLC6A4) was found, resulting in allelic variation of 5-HTT expression and function, and associated traits of negative emotionality including anxiety and depression (Collier et al., 1996; Lesch et al., 1996; Caspi et al., 2003, 2010; Canli and Lesch, 2007).