In a study exploring the biological connection between stress and PTSD, highly traumatized women exhibited elevated blood levels of pituitary adenylate cyclase-activating polypeptide (PACAP) and single-nucleotide polymorphisms in its receptor (PAC1R) gene, which were associated with a diagnosis of PTSD and with the extent of fear conditioning responses [107]. This evidence concerns the gene ADCYAP1 and post-traumatic stress disorder.