Panic disorder is a severe form of anxiety in which the affected individual feels an abrupt onset of fear, often accompanied by profound physical symptoms of discomfort. Scientists have known from studying twins that genes contribute to the risk of panic disorder, but very little is known about which specific genes are involved. Two of the most common and terrifying symptoms of this severe anxiety are a sense of shortness of breath and feelings of suffocation. One theory exists that panic disorder involves an overly sensitive “suffocation alarm system” in the brain that evolved to protect us from suffocating, and that panic attacks result when this alarm gets triggered by signals of impending suffocation like rising carbon dioxide levels.