Negative correlation between DNA methylation and CpGo/e ratio (the observed vs. the expected CpG ratios) in humans (A) and rhesus macaques (B). For each 2700 bp promoter region, the 540bp window closest to the transcription start site was used. The regions were grouped into bins based on their CpGo/e ratios, and at least 40 regions were required in each bin. The X-axis label of each bin represents the minimum CpGo/e value. For example, “0.2” refers to a bin with the CpGo/e values ranging between 0.2 and 0.3. The methylation levels were the average of 3 individual samples in each species.