Inclusion Criteria:  The diagnosis of MELAS syndrome is based on medical history (lactic acidosis, stroke-like episodes, and encephalomyopathy). Subjects have to be clinically stable for more than six months after any stroke-like episodes. All subjects have to be genetically confirmed.  Exclusion Criteria:  Subjects harboring a MELAS-related pathogenic mtDNA mutation, no fulfilling the complete diagnostic criteria for the MELAS phenotype.