For example, Nielsen and colleagues [19] have recently identified that methylation CpG sites in the apolipoprotein E (APOE) promoter region were related to the plasma APOE protein levels, and plasma APOE protein levels were associated with posttraumatic stress disorder symptom severity in a cohort of veterans with and without mild TBI (mTBI). The gene discussed is APOE; the disease is post-traumatic stress disorder.