Since DNA methylation of regulatory elements usually acts to repress gene transcription, the findings by Ressler and colleagues [70] indicating that methylation of Adcyap1r1 is associated with impaired fear extinction (as typically occurring in PTSD) together with the present ones indicating that the up regulation of this gene is associated with efficient fear extinction converge in highlighting that the system pituitary adenylate cyclase-activating polypeptide with its receptors might be an important mediator of abnormal fear responses following trauma [70]. This evidence concerns the gene ADCYAP1R1 and post-traumatic stress disorder.