Blood samples were collected for genetic analyses. Initial genotyping of 1608 subjects was performed by Perlegen Sciences using custom arrays and by the Center for Inherited Disease Research (CIDR) using Illumina Golden Gate technology as previously detailed [17–18, 23]. Genotyping of the additional 445 subjects was done at Washington University using Illumina Golden Gate technology and Sequenom MassArray iPLEX technology [23]. Self-reported race was verified using EIGENSTRAT [25]. For this study, we focused on four SNPs: rs16969968 and rs578776 located in the CHRNA5-CHRNA3-CHRNB4 gene cluster, rs13277254 in CHRNB3-CHRNA6, and rs12466358 in the CHRND gene. All four SNPs had call rates of 98% or better. Allele correspondence was checked for the combined genotyped samples.
