In humans, the promoter region of the SERT gene has a repetitive sequence insertion/deletion (IN/DEL) polymorphism, known as the SERT polymorphic region (SERTPR, or 5-HT transporter (5-HTT)-linked polymorphic region, 5-HTTLPR) that affects its expression level; it has been repeatedly indicated that this polymorphism is associated with many psychiatric states such as depression, anxiety, and suicidal behavior (Serretti et al., 2006; Canli and Lesch, 2007; Murphy et al., 2008). The gene discussed is SLC6A4; the disease is major depressive disorder.