The 9R polymorphism is particularly interesting in humans since its presence is associated with increased DAT expression levels (Michelhaugh et al., 2001; van de Giessen et al., 2009; Faraone et al., 2014) as well as post-traumatic stress disorder (Fuke et al., 2001; Segman et al., 2002; Valente et al., 2011; Hoexter et al., 2012). Here, SLC6A3 is linked to post-traumatic stress disorder.