Consistent with these findings in HET mice, adult 5-HTT full knockout (KO) mice (non-stressed) show increased anxiety-related behaviors (Carroll et al., 2007), higher depression-related behaviors when exposed to repeated stress and decreased fear extinction [(Wellman et al., 2007) see also (Carola and Gross, 2012; Homberg and Van Den Hove, 2012)]. Here, SLC6A4 is linked to major depressive disorder.