BDNF and post-traumatic stress disorder: Of those models based on well-known human polymorphisms, candidate genes associated with allele-specific variation in extinction (Lonsdorf and Kalisch 2011), include the BDNF Val66Met (see Felmingham et al. 2018 for evidence of a link between BDNF alleles and impaired fear extinction learning in PTSD) and COMT Val158Met polymorphisms (Table 1).