Despite the BDNF (rs6265) GG (Val/Val) genotype being most frequent in some of the different populations studied, genetic and environmental heterogeneity, including culture, are one of the factors that may lead to this noncompliance between the results and the controversy on whether the BDNF (rs6265) genetic variant and its serum protein levels are associated or not with MDD. Here, BDNF is linked to major depressive disorder.