BDNF and major depressive disorder: Stratified analysis of the patients with moderate (HAM-D<24) and severe (HAM-D≥24) MDD showed no significant correlation (p = 0.775) between BDNF serum levels and severity of disease in moderate MDD, while patients with severe MDD had a tendency (p = 0.064) towards a positive correlation between baseline BDNF levels and depression severity (Fig 1B).