This analysis was limited by the fact that we were not able to adjust for local admixture, and the chip coverage of common variants (>5%) is less complete compared with the European populations,51 which applies to most GWAS of African American populations. However, the use of a global adjustment for population genetic variation in the regression analysis using the principal components approach provided some measure of control for potential confounding because of population admixture.34, 52 Additionally, we acknowledge the limited precision of the smoking phenotypes. Smoking quantity is a highly heritable trait: estimates for CPD, heavy versus light smoking and/or pack-years range from 40 to 70% heritability in the European, African and Asian ancestry twin and family studies. Other studies have estimated that shared environmental factors account for 50% or more of the observed variation in SI, AOI and SC.1, 18, 20, 53, 54, 55, 56, 57
