Interestingly, two rare hereditary neurodegenerative diseases, the autosomal dominant cerebellar ataxia, deafness and narcolepsy (ADCA-DN) and the hereditary sensory neuropathy with dementia and hearing loss (HSN1E), whose phenotypes share some clinical features typical of mitochondrial diseases, including mitochondrial dysfunction, optic atrophy, peripheral neuropathy and deafness, have been linked to dominant pathogenic mutations in the DNMT1 gene [131]. The gene discussed is DNMT1; the disease is hereditary sensory neuropathy-deafness-dementia syndrome.