We further explored the effects of rs1996371 on smoking behaviors by testing whether this SNP modifies the AAO of habitual smoking among subjects who are already daily smokers. For non-habitual smokers, the AAO measures were right-censored at each subject's age at their most recent assessment. Our Cox-proportional analysis using AAO of habitual smoking censored at non-habitual smokers demonstrated that the individuals who have a minor allele of rs1996371 and highly correlated variants have 1.2 times more chance of becoming habitual smokers with every one-year increase in their age (Table 4). We tested the proportional hazard assumption for all covariates using Schoenfeld residuals and no violation was detected. The strongest effect of association with rs1996371 is between 14 and 21 years (Figure 1). At age 21, 43% of individuals with 2 copies of the minor allele of rs1996371 are habitual smokers while 25% of individuals homozygous for the major allele are habitual smokers (Figure 1). We also performed the same analysis using rs16969968 as a covariate and confirmed that the effect of rs1996371 and correlated variants on AAO of habitual smoking
