The prevalence, number of symptoms, and symptom severity of posttraumatic stress disorder (PTSD) by number of traumatic events and methylation beta-value. The results show that the association between number of traumatic events and PTSD diagnosis is modified by SLC6A4 methylation. Exposure to a greater number of potentially traumatic events (PTEs) is associated with increased risk of PTSD diagnosis (logistic), greater number of symptoms (linear), and greater severity of symptoms (negative binomial) at low levels of SLC6A4 methylation; in contrast, at high levels of SLC6A4 methylation, exposure to a greater number of traumatic events is associated with resilience to developing PTSD. Methylation beta values of <0.2 and >0.8 have previously been characterized as unmethylated and methylated, respectively[28].