This research study is being done to try to find a way to accurately diagnose post-traumatic stress syndrome (PTSD) in combat veterans.Diagnostic biomarkers have made invaluable contributions to the diagnosis and treatment monitoring of many diseases and disorders. Unfortunately, PTSD currently lacks a reliable and compelling clinically-relevant biomarker. The absence of a viable biomarker impairs efficient and confident diagnosis of PTSD, with diminished effective care options often resulting.