The study participants for the NAG project (Saccone et al., 2007a; Loukola et al., 2008) were enrolled at two different sites: the Queensland Institute of Medical Research in Australia, and the University of Helsinki in Finland. Families were identified through smoking index cases selected using previously administered interview and questionnaire surveys of the community-based Australian and population-based Finnish cohorts of twins. The Finnish arm of the NAG project recruited families of Finnish ancestry through twin pairs from the Finnish Twin Cohort, born between 1938 and 1957. Families chosen for the Australian arm of the NAG study were identified from two cohorts of the Australian Twin Panel, which included opposite-sexed twins and spouses of the older of these two cohorts, for a total of approximately 12,500 families with information about smoking. The ancestry of the Australian samples is predominantly Anglo-Celtic and Northern European (> 90%), while the Finnish sample is all of Finnish ancestry. Further details about these samples are given elsewhere (Saccone et al., 2007a). All data collection procedures were approved by institutional review boards at the lead institution, Washington
