nicotine also provides a plausible mechanism for explaining findings from three closely related investigations showing that risk of smoking and lung cancer is associated with a group of genes in chromosome 15 coding for alpha-3, alpha-5 and beta-4 nicotine receptor subunits [35–37]. In the aggregate, the nicotine receptor findings provide critically needed genotype specificity for twin studies, indicating that at least 50% of the variability in smoking initiation is due to genetic factors [26,27].
