In Ressler et al. the variables in the three scatter plots are: severity of post-traumatic stress disorder (PTSD) symptoms vs. serum concentration of pituitary adenylate cyclase activating polypeptide (PACAP, also known as ADCYAP1; Figure 2A); severity of PTSD symptoms vs. PACAP receptor gene methylation (also known as ADCYAP1R; Figure 2B); and abundance of PACAP receptor mRNA in the cerebral cortex vs. abundance of PACAP mRNA in the cerebral cortex (Figure 2C). Here, ADCYAP1 is linked to post-traumatic stress disorder.