Fully methylated DNA refers to a process of adding a methyl group to the DNA molecule. This sequence modification is critical in regulating gene expression, genomic stability and X-chromosome inactivation. There are several synonyms used for fully methylated DNA, including methylated DNA, DNA methylation, and DNA hypermethylation. DNA hypermethylation is a term used to describe the excessive methylation of cytosine residues that can lead to gene silencing. Methylated DNA refers to the process of attaching a methyl group to a DNA molecule. This process can occur at any of the DNA bases, although it most commonly is used to describe the methylation of cytosine residues. DNA methylation is a crucial regulatory mechanism that plays a significant role in various biological processes, such as embryonic development and cellular differentiation.