Susceptibility to PTSD is determined by both genes and envi-ronment. Similarly, gene-expression levels in peripheral blood are influenced by both genes and environment, and expression levels of many genes show good correspondence between pe-ripheral blood and brain. Therefore, our objectives were to test the following hypotheses: (1) pre-trauma expression levels of a gene subset (particularly immune-system genes) in peripheral blood would differ between trauma-exposed Marines who later developed PTSD and those who did not; (2) a predictive bio-marker panel of the eventual emergence of PTSD among high-risk individuals could be developed based on gene expression in How to Cite this Article: