Structured Abstract Importance Mitochondrial DNA (mtDNA) disorders have emerged as major cause of inherited neurological disease. Despite being well recognized for over two decades, the clinical presentation continues to broaden. The phenotypic heterogeneity is partly due to different percentage levels of mutant mtDNA heteroplasmy in different tissues, but the factors influencing this are poorly understood. Observations A case report describing monozygotic male twins with ptosis, optic atrophy, and recent onset intractable myoclonic epilepsy. The assessment of respiratory chain enzyme activities in muscle from one twin revealed a severe and isolated defect involving mitochondrial complex I. MtDNA sequencing revealed a pathogenic m.14487T>C MTND6 mutation, which was present at very high levels of heteroplasmy (84%) in muscle, and lower levels in blood (15%), urinary epithelium (75%), and buccal mucosa (58%). Of particular interest, his identical twin was found to harbor very similar levels of the m.14487T>C mutation in his blood, urine, buccal mucosa and hair follicle DNA samples, whilst the presence of low levels in the mother’s tissues confirmed maternal transmission. Conclusions and relevance m14487T>C can also cause the unusual combination of optic atrophy, ptosis, and encephalomyopathy leading to intractable seizures. Near-identical heteroplasmy levels in different tissues in both siblings supports a nuclear genetic mechanism controlling the tissue segregation of mtDNA mutations.