In this study, we have determined the degree of methylation at each of the 16 CpG sites in the upstream CpG island of the OPRM1 gene in 70 African-American and 53 Hispanic controls. We have previously reported the degree of methylation at each of these CpG sites in 103 Caucasian controls (Nielsen et al. 2009). When we compared the methylation levels of all the CpG sites in the controls of each ethnicity (Table 2), the African-Americans had a higher mean methylation level (19.6%) than was found in the Hispanics (16.1%) with an experiment-wise P = 3.48 × 10−6 or in Caucasians (17.4%) with an experiment-wise P = 0.0031. There was no experiment-wise difference in mean methylation levels between Hispanic controls and Caucasian controls.
