In the present study, we observed numerous GWS signals that have not been reported previously but none in the chromosome 15 cluster of genes encoding nicotinic receptors. This may be attributable to two features of our sample and the nature of the reported association. First, we had excellent representation of AA subjects, and it was from this part of the sample that the most interesting findings were derived. Second, our subjects were recruited for the purpose of studying other types of SD (opioid, cocaine, and alcohol dependence) without reference to cigarette smoking or other tobacco use. Thus, our sample had high affection with and comorbidity for other SD traits. Although comorbidity is often not reported in ND GWAS, we surmise that our sample was more severely affected with drug phenotypes than other, previously reported samples. We controlled for this comorbidity analytically.
