Subjects for this study were recruited at four sites: Yale University School of Medicine (APT Foundation; New Haven, CT), University of Connecticut Health Center (UConn; Farmington, CT), McLean Hospital (Harvard Medical School; Belmont, MA), and Medical University of South Carolina (MUSC; Charleston, SC). Subjects were originally ascertained for affected sibling pair linkage studies of the genetics of cocaine or opioid dependence (23, 24). Families were recruited based on screening suggesting that two siblings would meet diagnostic criteria for opioid dependence (at the UConn and Yale sites) or cocaine dependence (all sites). Alcohol use played no role in proband selection or pedigree extension. Subjects were classified as African- American (AA) based on a Bayesian model-based clustering method using genetic marker information, with STRUCTURE (28). We used 100,000 iterations for the burn-in, with a run length of 100,000. Included in the STRUCTURE analyses were 1408 SNPs from the genomewide scan. The program implements a model-based clustering method for inferring population structure using genotype data. Individuals in the sample are assigned (probabilistically) to populations, or jointly to two or more populations if their
