Post implantation, de novo methylation begins (red circles), mediated primarily by DNA (cytosine-5-)-methyltransferase-3alpha (DNMT3A) and -3beta (DNMT3B) (top). When methylation affects CpG ...

The latter is called an epigenetic alteration. While cytosine methylation is a healthy process, methylation of promoter CpG islands has also been shown to be a critical hallmark in many cancer cells.