BDNF and depressive symptom measurement: Suggestive evidence has been reported for associations between depression and candidate genes regulating the serotonin system (e.g., 5-HTTLPR, a functional polymorphism in the promoter region of the serotonin transporter gene SLC6A4), the hypothalamic-pituitary-adrenal (HPA) axis (e.g., CRHR1), or the neurotrophins axis (e.g., BDNF) [4, 5].