Candidate genes (and the SNPs in the corresponding gene regions) were grouped based on known functional similarity. We used a slightly modified version of the groups developed by the NICSNP Candidate Gene Committee. (Table S1). To test for association between SNPs in a group and smoking behaviors, we used a modified rank truncated product method [60] which compares the product of the ten smallest gene-level p-values over all the genes in the group to its simulated null distribution. Such a group or pathway level analysis potentially has more power to detect associations when a group containing multiple susceptibility genes each has modest evidence for association [61].
