Neurotransmitters such as serotonin IA receptor [15] and some neurotrophic factors including brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), glial cell line-derived neurotrophic factor (GDNF), and vascular endothelial growth factor (VEGF) [16,17,18,19] have been investigated as potential biomarkers in psychiatric diseases. The gene discussed is GDNF; the disease is psychiatric disorder.