BDNF and major depressive disorder: In MDD, many previous magnetic resonance imaging (MRI) studies using the voxel-based morphometry (VBM) technique have reported evidence of a relationship between brain volume and genetic factors, including brain-derived neurotrophic factor (BDNF), norepinephrine transporter (NET) gene (SLC6A2), and methylenetetrahydrofolate reductase (MTHFR)/catechol-O-methyltransferase (COMT) polymorphisms16–22.