This prospective cohort study will compare breath biomarkers from subjects with mitochondrial disease to healthy controls. The age range for the study is 4-80 years and all subjects will be recruited at CHOP. Participation would consist of a one time visit, up to 45 minutes maximum, to collect identifiable information and breath sample. The goal is to identify VOC biomarkers that characterize mitochondrial disease compared to healthy controls, which could lead to a non-invasive breath test for mitochondrial function. Subjects will be compensated as a form of appreciation for participating in the study.