BDNF and anxiety disorder: Some studies have demonstrated a link between brain volume and genetic polymorphisms in PTSD, for example, the brain-derived neurotrophic factor (BDNF) Val66met polymorphism may increase susceptibility to PTSD and anxiety disorders via an interaction with reduced ventromedial prefrontal cortex and insular cortex volume [19], the FK506-binding protein 5 (FKBP5) rs1360780 is associated with smaller gray matter volumes in the dorsal anterior cingulate cortex [20], and the catechol-O-methyltransferase (COMT) polymorphism moderates the association between PTSD and hippocampal volume [21].