Definition of smoking-related phenotypes was essentially the same as in our previous study on the investigation of association of CHRNA5/A3/B4 with smoking in the same sample (Li et al. 2010). Briefly, on the basis of the survey questionnaire, from which information on smoking status (i.e., never smoker, former smoker who quit one year before being enrolled in the study, occasional smoker who smoke irregularly or only on specific occasions such as when drinking, or habitual smoker who smoke regularly on a daily basis) and number of cigarettes smoked per day (CPD) for the habitual smoker was drawn, three smoking-related phenotypes were defined for SI and ND. Regarding SI, the first measure (called SI-1) was defined as a binary trait comparing “never smoked” and “having regular smoking experiences (former, occasional, or habitual smoker),” and the second measure (called SI-2) was defined as an ordinal trait with four categories: never, former, light, and habitual. The ND phenotype was defined as an ordinal trait with five categories according to CPD: non-smoking, <10 CPD, 11–20 CPD, 21–30 CPD, and >31 CPD. Such assessment of
