Taken together, these data suggest that neurotrophin-mediated neuroplasticity in brain regions of critical importance for incorporating traumatic and stressful memories, e.g., hippocampus, amygdala and pre-frontal cortex, contribute to PTSD and associated stress-related disorders (e,g., depression, anxiety), and that circulating levels of BDNF and NGF may reflect these brain alterations. This evidence concerns the gene BDNF and major depressive disorder.