Furthermore, there are data about the association between depression and specific variants of genes involved in serotonin synthesis and signaling, including TPH2 (Zill et al., 2004), serotonin receptors (Lemonde et al., 2003), and serotonin transporter (SERT) (Hoefgen et al., 2005; Avshalom et al., 2010). Here, SLC6A4 is linked to major depressive disorder.