BDNF and major depressive disorder: When observing only the BDNF Val66Met (rs6265) genetic variant frequency and genotypic distribution (Table 1), the GG (Val/Val) genotype was frequent in more than 50% of the MDD sample studied in 65% of the analyzed studies [17, 18, 19, 21, 23, 25, 26, 30–33] (Figure 3).