One change in the sequence of the BDNF gene causes some people to be more impaired by traumatic brain injury than others with comparable wounds, new research shows. The study measured general intelligence in a group of 156 Vietnam War veterans who suffered penetrating head injuries during the war. All of the study subjects had damage to the prefrontal cortex, a brain region behind the forehead that is important to cognitive tasks such as planning, problem-solving, self-restraint and complex thought. The team found that a single polymorphism (a difference in one “letter” of the sequence) in the BDNF gene accounted for significant differences in intelligence among those with similar injuries and comparable intelligence before being injured.