CSF Brain Derived Neurotrophic Factor (BDNF) levels have been found to be similar in PD patients with major depression to those in patients with major depression without PD in one study (Pålhagen et al., 2010), while another described this value as increased in PD patients compared with controls (Salehi and Mashayekhi, 2009). This evidence concerns the gene BDNF and major depressive disorder.