Particularly in the case of Rett Syndrome, the most common genetic cause of severe intellectual disability in females, several studies in animal models of the disease have demonstrated that animal do not develop an irreversible condition and that phenotypic rescue may be possible, highlighting the need to understand the biological role of MeCP2 and particularly its involvement in the regulation of DNA methylation in the brain [72]. The gene discussed is MECP2; the disease is atypical Rett syndrome.