Among them, the presence of a polymorphism of either SERT (Bengel et al., 1998; Kuzelova et al., 2010), 5-HT1A receptor (Lemonde et al., 2003), the tryptophan hydroxylase type 2 (TPH-2; Invernizzi, 2007), or BDNF (Chen et al., 2006) is associated with the occurrence of depression related to stress, or to a response to behavioral tests predictive of the antidepressant-like activity of a molecule (Porsolt et al., 1977; Steru et al., 1985). The gene discussed is TPH2; the disease is major depressive disorder.