BDNF and post-traumatic stress disorder: A plethora of studies demonstrate a connection between the val66met polymorphism of BDNF and PTSD in relation to: extinguishing the fear and startle response (Rattiner et al., 2004; Zhang et al., 2014); PTSD symptomology and severity (Koenen et al., 2009; Frielingsdorf et al., 2010; Hemmings et al., 2013b); psychotic PTSD (Pivac et al., 2012); and the efficacy of PTSD therapy (Felmingham et al., 2013).