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 2 hours 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.