There are many environmental factors such as influence of peer group [28], lack of parental monitoring [29] and easy access to cigarettes can also influence the genetic liability to smoking behavior in young adults. Koopmans et al [30] reported that the total variance accounted by genetic influences on smoking initiation and cigarettes per day is 39% and 86%, respectively. The liability to ever smoking is largely independent from liability to continue to smoke. This could be due to the fact that while over 80% of individuals in some studies report that they have tried smoking cigarettes, only about one third of them will smoke beyond experimentation [31]. Hence genetic factors may have stronger impact on daily smoking (smoked ≥4 days/week) and habitual smoking (one pack a day for 6 months) rather than smoking initiation. Therefore in the present study we focused on subjects ascertained as daily smokers to find out the relationship between CHRNA5-CHRNA3-CHRNB4 variants and onset of daily/habitual smoking behaviors, believing that this selected group can explain more genetic variance than smoking initiation or exposure. We found that the
