Data collection has been described in previous publications (Broms et al. 2007; Loukola et al. 2014). Briefly, subjects were ascertained from the Finnish Twin Cohort study encompassing 35,834 twins born in 1938–1957. Twin pairs concordant for smoking were recruited, along with their family members (mostly siblings). Altogether 1962 subjects (mean age 56.2 ± 8.3, 50.9% men) from 734 families were included in this study, consisting of 858 subjects from 429 full dizygotic (DZ) pairs, 146 additional DZ subjects (one co‐twin per pair), 131 monozygotic (MZ) subjects (one co‐twin per pair), 19 twin subjects (unknown zygosity) without co‐twins, 681 siblings (brothers and sisters), and 127 parents (one parent per family). All the subjects had initiated smoking and smoked on average 15 CPD. Altogether 880 subjects were successful quitters defined by self‐reported abstinence of at least 6 months at the time of the interview. Written informed consent was obtained from all subjects who were interviewed and/or gave DNA samples before the beginning of the studies. The collection of the informed consent as well as blood samples followed the recommendations given in the
