The PedCheck program was used to determine genotyping consistency for Mendelian inheritance of all 11 SNPs selected for replication. To verify the quality of our genotyping, we also checked the SNP data for any significant departure from HWE. The HWE at each locus was assessed by the χ2 test. The allele frequencies for each genetic marker were calculated using the FREQ program of S.A.G.E. (v. 5.0). Associations between individual SNPs and smoking behavior phenotypes, including the ND measures and age at onset, were determined by the PBAT program using generalized estimating equations (Lange et al. 2003). Three genetic models (additive, dominant, and recessive) were tested, with sex and age as covariates in the AA and EA samples; sex, age, and ethnicity served as covariates for the combined sample. Relations with the three ND measures and age at onset were examined individually.
