Figure 4.

DNA methylation in normal and cancer cells. CpG dinucleotides present at a low density within the genomic sequence are mostly methylated (e.g., the silencing of transposable elements and the condensation of centromeric and subtelomeric regions). On the other hand, a small proportion of CpG dinucleotides in clusters named CpG islands are often found within the gene promoter regions. These CpG islands are rarely methylated in healthy cells. Aberrations in the epigenetic information in cancer cells are characterized by global genomic hypomethylation accompanied by aberrant hypermethylation of the CpG islands present in the promoter regions of tumor suppressor genes. The red arrow with the cross indicates the repression of transcription; the green arrow indicates active transcription. The green rectangle represents a promoter site for tumor suppressors and the gray rectangle shows a promoter site for oncogenes.