Polymorphisms in several serotonin receptor genes, such as HTR1A, HTR3A, and HTR2C, have been shown to interact with stressful life environment as well as polymorphisms from other genes (e.g., Val66Met in the BDNF gene) to predict susceptibility to depression (Kim et al., 2007, 2011a; Gatt et al., 2010), and to mediate HPA-axis activation and emotional response to stress (Brummett et al., 2012). Here, BDNF is linked to major depressive disorder.