Mutations in the Slc6a4 promoter affect the rate of serotonin uptake (Caspi et al., 2003) and are therefore associated with numerous diseases like sudden infant death syndrome (Weese-Mayer et al., 2003), and propensity to post-traumatic stress disorder (Liu et al., 2015) or susceptibility for depression (Lin et al., 2015). Here, SLC6A4 is linked to major depressive disorder.