Research using a knock-in mouse model containing the human variant Val66Met (rs6265) in the gene coding for brain-derived neurotrophic factor (BDNF), as well studies in human carriers with this BDNF variant has shown that both mice and human carriers exhibited impaired fear extinction [14], which may be a specific biomarker of PTSD treatment response [15]. This evidence concerns the gene BDNF and post-traumatic stress disorder.