Genotyping was conducted with a modified single nucleotide extension reaction, with allele detection by mass spectrometry [Sequenom MassArray system; Sequenom, San Diego, CA]. PCR and extension primers (available upon request) were designed using MassARRAY Assay Design Version 3.1.2.5. In the CDP sample, we selected nine SNPs in CHRM2 for genotyping, based on evidence of association with one or more externalizing disorders in the COGA sample (Dick et al., 2007a; Wang et al., 2004); Figure 1 depicts the locations of the SNPs, which spanned 81,130 base pairs between intron 3–4 and the 3′ untranslated region of the gene. The overall genotyping success rate was 98.4%. Twenty-four biological and 12 technical replicates were genotyped, and produced a concordance rate of 100%. All 9 SNPs were successfully genotyped for 96% of the samples.
