Though the underlying neurobiological dysfunctions in both MDD and in the residual symptoms of MDD remain to be fully elucidated, potential biological factors associated with residual symptoms may include genetic variability of CYP450 genes, expression of brain-derived neurotrophic factor (BDNF), or serotonin 5HT transporter density, as well as underlying disturbances in the hypothalamic-pituitary-adrenal (HPA) axis [57]. This evidence concerns the gene BDNF and major depressive disorder.