Hypothesis and mechanism-focused candidate gene studies have identified PTSD-related differences in DNAm levels in genes associated with the hypothalamic-pituitary-adrenal (HPA) axis (e.g., ADCYAP1 [9], FKBP5 [10], and NR3C1 [11]), inflammation (e.g., BDNF [4], HTR2A [12] and IL-18 [13]), and neurotransmission (e.g., BDNF [4], HTR2A [14], and HTR3A [15]). Here, BDNF is linked to post-traumatic stress disorder.