For example, ghrelin was reported to both promote (Carlini et al., 2002, 2004; Hansson et al., 2011; Currie et al., 2012; Spencer et al., 2012; Meyer et al., 2014) and alleviate (Lutter et al., 2008; Spencer et al., 2012; Jensen et al., 2016; Huang et al., 2017) anxiety- and depression-like behaviors, a dual but disparate effect which may depends on both contextual states (for example, non-stress vs. stress, acute vs. chronic stress, mild vs. strong stress) and physiological states (for example, food availability) of the animal (Spencer et al., 2015). The gene discussed is GHRL; the disease is major depressive disorder.