Significantly lower levels of serum BDNF were found in antidepressant-free patients with major depressive disorder compared with healthy controls (Shimizu et al. 2003), which findings were confirmed by a large cohort study (Molendijk et al. 2011) and by three meta-analyses as well (Bocchio-Chiavetto et al. 2010; Brunoni et al. 2008; Sen et al. 2008). This evidence concerns the gene BDNF and major depressive disorder.