After the complete sequencing of the human genome, one of the next important steps is to decipher the genetic mechanisms which allow cells to switch genes on and off. These processes are fundamental for cellular development. At the chemical level, gene silencing requires the selective methylation of of the C5-position of cytidines in DNA, and this established method is today known as the epigenetic control of gene function. Failure of the epigenetic control of genes has been connected to many human diseases, including neurodevelopmental disorders and cancer. The identification of 5-methylcytosine residues (5MedC) in genes is consequently of paramount importance for the diagnosis of modern diseases.