2. Smokers. A "smoker", for the purposes of the study, will be defined as an individual who has regularly and habitually used nicotine-containing substances, including tobacco products (e.g., cigarettes, cigars, chewing tobacco, gum, patch or electronic cigarettes) over the past 12 months. Occasional recreational use (less than once monthly) will not warrant exclusion unless the individual has used nicotine-containing substances in the previous 30 days before study enrollment.