It should be noted that patients with MDD show increased Hdac2 expression and reduced Gdnf expression in the peripheral blood cells and whole blood (Takebayashi et al., 2006; Otsuki et al., 2008; Hobara et al., 2010), supporting a contribution of the HDAC2–GDNF pathway to the depression. Here, GDNF is linked to major depressive disorder.