Stress is a necessary physiological response to external pressures on homeostasis. However, extreme or chronic exposure, i.e. trauma, can lead to debilitating disorders such as Post-Traumatic Stress Disorder (PTSD). Symptoms of PTSD include nightmares of the trauma, general cognitive dysfunction, changes in arousal, and avoidance behavior. Previous research has demonstrated that 30-70% of the variation in PTSD can be explained by heritability. Recent work shows that the genetic factor, apolipoprotein E (apoE) may differentially influence PTSD susceptibility and symptom severity. ApoE helps regulate lipid transport and metabolism throughout the body, including the brain. It exists in humans as three isoforms: E2, E3, and E4. Both mouse models and human studies have shown that carrying the E2 allele corresponds to greater PTSD-related symptoms and in humans, greater susceptibility after trauma. However, other studies have found conflicting evidence and suggest the association between apoE isoform and PTSD is still unclear.