This SNP (rs279858) encodes a silent polymorphism in exon 5 and consists of alleles G (minor) and A (major). In our sample, the minor allele frequency was 37.8 % in non-Hispanic Caucasians and 42.3 % in Hispanics. Similar to previous work, we combined non-Hispanic Caucasian and Hispanic participants in analyses because both groups had the same minor allele and similar minor allele frequencies [13]. Ancestry informative markers controlled for population stratification (see below). Based on a previous study using a similar sample in terms of age, ethnicity, and high risk for substance use disorders, we coded the SNP such that those with the AA and AG genotypes were coded 0 and the GG genotype coded 1 [10]. Genotype did not differ by ancestry (see Table 2).
