SLC6A4 and Anxiety: 5-HTT −/− mice are characterized by a robust increase in anxiety-like behavior, as well as a decrease in exploratory locomotion (Holmes et al., 2003; Carroll et al., 2007; Heiming et al., 2009), while 5-HTT +/− mice only display these behavioral traits when they have experienced experimentally induced adversity (Carola et al., 2008; Jansen et al., 2010; van den Hove et al., 2011).