Polymorphisms in the dopamine receptor genes, including DRD2 and DRD4, and in the dopamine transporter gene DAT1, have also been implicated in stress responsivity, emotion processing, and susceptibility to PTSD and depression (Segman et al., 2002; Dunlop and Nemeroff, 2007; Blasi et al., 2009; Ptacek et al., 2011; Skelton et al., 2012). Here, SLC6A3 is linked to major depressive disorder.