BDNF and depressive symptom measurement: Although not all studies reported positive or convergent findings, meta-analyses support the view that carrying the low-expressing alleles of the Val66Met polymorphism in the brain-derived neurotrophic factor (BDNF) gene [45] and the 5-HTTLPR polymorphism in the serotonin transporter gene promoter [46,47] is associated with increased depression and anxiety in people with a history of stressful events.