Methods: This is a pilot study and is part of the Newcastle Mitochondrial Research Biobank. Stool samples will be collected from patients with a Mitochondrial Encephalomyopathy Lactic Acidosis and Stroke-like episodes (MELAS) phenotype carrier of the m.3243 A>G mutation (N=20) from the United Kingdom Medical Research Council (MRC) Centre for Mitochondrial Disease Patient Cohort (RES/0211/7552, the largest cohort of mitochondrial patients in the world) and the mitochondrial clinic and age and gender matched healthy controls (N=20). DNA will be extracted from stool samples and the 16S rRNA gene (V4 region) will be sequenced. This data will be analysed using bioinformatics pipelines and computational biology.