Relationship between DNA methylation and gene expression for genes with different promoter CpG levels. The four panels show the expression levels of different subsets of genes and their corresponding methylation levels at upstream (A and B) or transcribed regions (C and D). Panels A and C involve the use of the mCG methylation measure, while panels B and D involve the use of the mCG/CG measure. Within each panel, the genes are first divided into three subsets according to their promoter CpG levels, which correspond to three bar groups. For each subset, the genes are further divided into another level of three subsets based on their methylation level. Finally, for each of the resulting subset of genes, their distribution of expression levels is shown by a Box and Whisker plot.