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Military missions can expose service members to horrible and life-threatening situations. As a result, 20 to 30 percent of combat veterans experience post-traumatic stress disorder (PTSD) – ongoing feelings of guilt, anger, and shame that surface as outbursts, reckless behavior and intense flashbacks. But despite PTSD’s prevalence among vets and other survivors of traumatic experiences, scientists know little about the condition’s biological roots, a gap tha