Based on our work in PTSD populations and previous studies of COMT genetic variation in fear extinction (Lonsdorf et al., 2009), we hypothesized that COMT Met158 homozygotes would display increased fear-potentiated startle to a safety signal and during extinction, and that this deficit in fear inhibition would be associated with PTSD. The gene discussed is COMT; the disease is post-traumatic stress disorder.