Researchers have found genetic test markers that indicate whether or not someone has post-traumatic stress disorder (PTSD), and a simple blood test could help find that out, they believe.

A team of international researchers is trying to track down a genetic market linked to PTSD in blood samples in Marines that are stationed in conflict zones, which will provide them with a way to determine who is most at risk for PTSD and who is better able to resist it, according to a CNN report.

PTSD is a tricky condition that plays on the brain, and can result after just about any type of trauma, whether it be rape, torture, natural disasters, or child abuse, according to the report. About 13 million Americans have PTSD.

The symptoms can include: reliving the event by way of nightmares which can results in chills or shaking; becoming emotionally withdrawn and detached from family and friends; become easily startled or angry and having difficulty sleeping.

People who have served in the armed forces and have been through a trauamatic attack, such as having their convoy bombed by an improvised explosive device or being in a gunfight, are often the hardest hit. The Department of Veteran Affairs estimates that about 30 percent of those who served Vietnam have suffered from PTSD, as well as 20 percent of those who served in a given year in Iraq or Afghanistan.

The new study shows that genes have a bigger role than people think in the development of PTSD. Researchers found that in mice, the lack of a fear-regulation brain chemical call gastrin-releasing peptide created a bigger fear response, and in other study, a protein that helps develop fear-based memory was lacking in mice that were better able to handle PTSD-like symptoms.

Also, studies of twins indicate that there is a hereditary responses, as identical twins were more likely to develop PTSD that fraternal twins.