To provide a (hetero)aryl-p-quinone derivative for a treatment of mitochondrial diseases.

Methods of treating or inhibiting mitochondrial diseases, for example friedreich ataxia (FRDA), Leber's hereditary optic neuropathy (LHON), mitochondrial myopathy, encephalopathy, lactacidosis, apoplexy (MELAS) and Kearns-Sayre syndrome (KSS), and compounds useful in the present methods, for example 2-(3-hydroxy-3-methylbutyl)-6-(hetero)aryl-p-quinone or 2-(3-hydroxy-3-methylbutyl)-3-(hetero)aryl-p-quinone derivatives are disclosed. Energy biomarkers useful in the case of evaluating a metabolic state of subjects and effects of a treatment are also disclosed. Methods of modulating, normalizing or enhancing the energy biomarkers, and compounds useful for such methods are also disclosed.