The addition of methyl groups to DNA can repress, or silence, gene expression by leading to a more compact DNA structure that excludes the binding of most proteins. Because of this, regions of DNA that are heavily methylated are not usually accessible to the binding of proteins needed for gene expression, such as transcription factors. Transcription repression is also aided by proteins that specifically bind to methylated DNA and contribute to the more compact DNA structure. These methyl-binding proteins contain a methyl-binding domain (MBD) that specifically recognizes methylated DNA. MeCP2, which causes a genetic disorder known as Rett syndrome, is one of these methyl-binding proteins that can bind to a single methylated cytosine in DNA and prevent the binding of other proteins like transcription factors. If it appears in a gene promoter region, it can prevent transcription from occurring.