A well-defined phenotype is crucial to the success of genetics research. Smoking cessation, the focus of this review, is simply defined as the ability to maintain long-term abstinence (Silagy et al. 2004). Effective cessation occurs by overcoming different milestones to achieve persistent abstinence, including a period of initial abstinence (24 h without smoking), avoiding lapse (first use after abstinence), and ultimately avoiding relapse (proceeding from first use after abstinence back to regular daily use) (Shiffman et al. 2006). The inability to maintain abstinence is related to the severity of withdrawal symptoms as well as genetic factors and influenced, in part, by psychological factors and contextual cues (Dawkins et al. 2009; Pomerleau et al. 2005; Xu et al. 2008).
