There have been numerous studies and reviews covering the role of 5-HTT in psychiatric disorders, indicating that polymorphisms in the 5-HTT lead to serotonergic dysfunction that can develop into various diseases such as major depressive disorder and bipolar disorder (Mann et al., 2000; Zanardi et al., 2000; Hahn and Blakely, 2002; Anguelova et al., 2003; Murphy et al., 2004; Abdolmaleky et al., 2014). Here, SLC6A4 is linked to major depressive disorder.