All study participants (N = 2,038) were recruited by the Collaborative Genetic Study of Nicotine Dependence (COGEND), a United States multi-site project based on community samples from St. Louis, MO and Detroit, MI [17–18, 23]. Cases and controls were required to have smoked at least 100 cigarettes lifetime, the threshold classically used to define a smoker. Cases were nicotine dependent, defined by current Fagerström Test for Nicotine Dependence (FTND) score of 4 or more [24]. Controls were defined as smokers (smoking at least 100 cigarettes lifetime), but who never had any symptoms of dependence (lifetime FTND=0). By selecting controls who smoked, we focused on those genetic and environmental effects that are specific to the development of nicotine dependence rather than smoking initiation. All selected COGEND participants were of European ancestry. See Table 1 for sample characteristics.
