Stress is a risk factor for anxiety disorders, but not all individuals exposed to stress develop anxiety. Differences in stress susceptibility can be explained by genetic and environmental factors rendering some individuals more prone to develop anxiety following stress than others. Stress influences anxiety via its effect on the brain. To understand why some individuals are more vulnerable to stress, we need to understand genetic and environmental influences on stress-related brain function. Our first aim is to determine genetic and environmental effects on brain function during stress. Stress also affects the immune system, which in turn could have long-term effects on brain function. Genes expressed by immune cells could produce compounds that affect brain function long after stress. Hence, stress susceptibility could depend on how genes and environment modulate stress induced gene-expression. Our second aim is to determine genetic and environmental influences of stress on gene expression in blood. Because we want to know how stress influences anxiety, our third aim is to establish which brain systems and gene transcripts are associated with elevated subjective ratings of anxiety during stress. Results will be important for uncovering novel pathways that could explain how stress causes anxiety in some individuals but not in others.