BDNF level was significantly decreased in patients with acute MDD in serum-based studies (Cohen’s d = −0.81; 95% CI = −1.05 to −0.56; p < 0.0001; I2 = 91.5%; N = 32, n = 2,298), but not in plasma-based studies (Cohen’s d = −0.71; 95% CI = −1.55 to 0.13; p = 0.097; I2 = 91.1%; N = 6, n = 149); this may be due to the smaller sample size. This evidence concerns the gene BDNF and major depressive disorder.