Tobacco use is a leading cause of preventable diseases in the world and causes nearly 5 million tobacco-related deaths annually [1]. According to the World Health Organization, 1.3 billion people are smokers and half of the smokers in the world today will die of smoking related diseases [2]. Nicotine is the primary factor responsible for the addictive behaviors. In last two decades, family, twin and adoption studies have implicated that genetic factors strongly influence the behaviors of tobacco use and nicotine dependence [3,4]. In the review by Sullivan and Kendler, they estimated that genetic factors accounted for 56% of the variance in the liability of smoking initiation (SI) and 67% of the variance for progression to nicotine dependence (ND) [4].
