All the participants involved in this study were recruited primarily from the mid-south states of Tennessee, Mississippi and Arkansas in the USA during 1999–2004, and are of either AA or EA origin. Proband smokers were required to smoke an average of 20 or more cigarettes per day for the last 12 months, to have smoked for at least the last 5 years, and to be at least 21 years of age. Once a smoker proband was identified, all his or her siblings and biological parents were recruited whenever possible, regardless of their smoking status. Participants included 1,366 individuals from 402 AA families and 671 individuals from 200 EA families, yielding a total of 2,037 individuals. The families varied in size from two to nine with an average size of 3.14 (±0.75; SD) for AAs and 3.17 (±0.69; SD) for EAs. Average age was 39.4 (±14.4; SD) years for the AA and 40.5 (±15.5; SD) years for EA participants. For more detailed demographic and clinical characteristics of this study, please refer to our previous reports (Li et al. 2005, 2006). All participants provided informed consent. The study protocol, forms and procedures were approved by all participating Institutional Review Boards.
