DNA methylation is mediated by DNA methyltransferases (DNMTs). Methylated cytosines have an impact on gene expression when they are located in promoter regions of genes. When present, this modification prevents transcription factors (TF) from binding to promoter regions and starting transcription and thus downstream gene expression. In the absence of DNA methylation (unmethylated cytosine), transcription factor can bind to the DNA strand and transcription of the gene concerned can proceed