Previous research has shown disparities in BDNF serum levels between BDNF Met+ vs. Met- participants, with increased levels shown in a study of 114 healthy controls (Lang et al., 2009), decreased levels demonstrated in 122 participants (66 healthy controls; 56 major depressive disorder, Ozan et al., 2010) and no difference found in a group of 198 participants [99 healthy controls; 99 amnestic mild cognitive impairment (aMCI), Yu et al., 2008]. This evidence concerns the gene BDNF and major depressive disorder.