The COGEND sample consists of individuals aged 25-44 who were recruited through telephone screening in St. Louis, Missouri and Detroit, Michigan. Subjects included 1054 nicotine dependence cases and 978 controls selected from the COGEND study who self-identified as being of European descent (Bierut et al., 2007). Nicotine dependent cases were defined as current smokers with a Fagerström Test for Nicotine Dependence (FTND) score of 4 or more (maximum score of 10; Heatherton et al., 1991). The FTND is an ordinal scale commonly used to measure nicotine dependence. Controls were defined as smokers (individuals who smoked at least 100 cigarettes lifetime), who never experienced any symptoms of dependence (lifetime FTND=0 or 1). The threshold of 100 cigarettes smoked over the lifetime is a commonly used threshold for significant smoking exposure (Bondy et al., 2009). By selecting controls who smoked, we focused our study on those genetic effects specific to the development of nicotine dependence. Only subjects of European American descent were included to decrease the genetic heterogeneity. The study was approved by the Institutional Review Board at each data collection site, and subjects provided informed consent prior to participating.
