A genetic discovery could lead to the first drugs designed to treat post-traumatic stress disorder (PTSD), say scientists.
The condition is treated with anti-depressants and anti-anxiety medication. Neither is very effective.
The new findings, published in the journal Archives of General Psychiatry, point to a promising therapeutic target for treating PTSD.
They suggest that gene variants that affect levels of a particular brain protein appear to play a key role in the condition.
Serotonin 1B receptors are molecules that trigger physiological effects when stimulated. One of their main functions is regulating levels of the "mood" chemical serotonin.
People susceptible to PTSD have fewer of the receptors than others, the research showed.
Alexander Neumeister, from Mount Sinai School of Medicine in New York City, said: "Our research ... sets the stage for the development of therapies that target serotonin 1B receptors, offering the potential to minimise the disabling effects of PTSD."
The study involved 69 people who had been victims of childhood abuse, domestic violence and military conflict. Of these, 49 suffered symptoms of PTSD.
Another 20 were exposed to trauma but did not have PTSD.
Serotonin 1B levels were substantially lower in patients diagnosed with PTSD.