BDNF and major depressive disorder: More specifically, the relationships of the BDNF Val66Met polymorphism with cognition (Egan et al., 2003; Hariri et al., 2003; Pezawas et al., 2004; Ho et al., 2006; Montag et al., 2009), emotion (Chen et al., 2006) and even with several brain disorders including major depression (Dalby et al., 2013; Choi et al., 2015), epilepsy (Chen et al., 2016), schizophrenia (Ho et al., 2006), and stroke (Ramos-Cejudo et al., 2015) are suggested to be mediated by its effects on the alterations in gray and white matters.