SLC6A4 and major depressive disorder: Knowing the large number of KO mice generated to date, it is not possible to detail the findings of each putative model interesting in the anxiety and depression field of research (SERT-/- mice, Bengel et al., 1998; NK1 receptor KO mice, Froger et al., 2001; Guiard et al., 2004; β-arrestin 2 KO mice, Beaulieu et al., 2008).