The distribution of DNA methylation patterns among the three CpG promoter density profiles (Fig. 2) is remarkably similar across species and tissue types,53,54 indicating that methylation pattern across CpG density is evolutionarily well-conserved from mouse to human. The fact that the methylation profiles are eliminated and reprogrammed twice in the life-cycle of each organism implies that maintaining methylation stability is crucial for establishing gene expression programs that are essential for functional maintenance of an organism. Also worthy of mention, in the LCP group, the methylation level increased with increasing CpG content (Fig. 2C), suggesting a relationship between CpG dinucleotide availability and DNA methylation levels. In the ICP group, a bell-shape distribution of methylation was observed, with the highest methylation levels at 0.02 CpG/bp (middle of the plot). In the HCP group, representing a majority (~60%) of the genes, the greater divergence of methylation was at lower CpG levels, and the methylation levels were reduced to the genome-wide median level as the CpG content increased (Fig. 2C). Most of the hypermethylated promoters in HCP have a lower CpG content, an observation
