A compound which is of formula I: where R is selected from the group consisting of: where the * indicates the point of attachment of R to the remainder of the molecule; n is 0 or 1; R 1 and R 2 are independently selected from the group consisting of hydrogen, (C 1 -C 6)alkyl, and (C 1 -C 6)alkoxy; R 3 is (C 1 -C 6)alkyl or (C 1 -C 6)alkoxy; R 4 is hydrogen, (C 1 -C 6)alkyl, hydroxy, or (C 1 -C 6)alkoxy; Het is optionally substituted heterocyclyl; M and M' are independently selected from the group consisting of hydrogen, -C -R 10,-C (C 2 -C 6)-alkenyl, -C (C 2 -C 6)-alkynyl, -C -aryl, -C -heterocyclyl, -C O-R 10, -C NR 10a R 10b, -SO 2 OR 10, -SO 2 -C 1 -C 6 -alkyl, -SO 2 -(C 1 -C 6)-haloalkyl, -SO 2 -aryl, -SO 2 -NR 10a R 10b, -P (OR 10a)(OR 10b), and C-linked amino acid or di-peptide, and R 10, R 10a, and R 10b are independently hydrogen or (C 1 -C 6)-alkyl optionally substituted with-OH, -NH 2, -NH(C 1 -C 4 alkyl), -N(C 1 -C 4 alkyl) 2, -C OH, -C O-(C 1 -C 4)-alkyl or halogen; or a salt, a solvate, or a hydrate thereof; for use in the treatment of a disorder selected from the group consisting of an inherited mitochondrial disease; Myoclonic Epilepsy with Ragged Red Fibers (MERRF); Mitochondrial Myopathy, Encephalopathy, Lactacidosis, and Stroke (MELAS); Leber's Hereditary Optic Neuropathy (LHON); Leigh Disease; Kearns-Sayre Syndrome; Friedreich's Ataxia; CoQ10 deficiency; cardiomyopathy, encephalomyopathy or other myopathy; renal tubular acidosis; a neurodegenerative disease; Parkinson's disease; Alzheimer's disease; amyotrophic lateral sclerosis; a motor neuron disease; a neurological disease; epilepsy; a genetic disease; Huntington's Disease; a mood disorder; schizophrenia; bipolar disorder; a cerebral vascular accident; macular degeneration; diabetes; cancer; oxygen poisoning; a sickle cell disease, thalassemia, or other haemoglobinopathy; energy disruption in the function of red cells; a hearing or balance impairment; a vision impairment; chronic progressive external ophtalmoplegia (CPEO); Complex I deficiency; Complex II deficiency; Complex III deficiency; Complex IV deficiency; Complex V deficiency; a respiratory chain protein disorder; stroke; ischemic stroke; hemorrhagic stroke; seizures; excitotic neuronal injury; optic neuropathy; dominant inherited juvenile optic atrophy; optic neuropathy caused by toxic agents; glaucoma; Stuargardt's macular dystrophy; diabetic retinopathy; diabetic maculopathy; retinopathy of prematurity; and ischemic reperfusion-related retinal injury. where the * indicates the point of attachment of R to the remainder of the molecule; n is 0 or 1; R 1 and R 2 are independently selected from the group consisting of hydrogen, (C 1 -C 6)alkyl, and (C 1 -C 6)alkoxy; R 3 is (C 1 -C 6)alkyl or (C 1 -C 6)alkoxy; R 4 is hydrogen, (C 1 -C 6)alkyl, hydroxy, or (C 1 -C 6)alkoxy; Het is optionally substituted heterocyclyl; M and M' are independently selected from the group consisting of hydrogen, -C -R 10,-C (C 2 -C 6)-alkenyl, -C (C 2 -C 6)-alkynyl, -C -aryl, -C -heterocyclyl, -C O-R 10, -C NR 10a R 10b, -SO 2 OR 10, -SO 2 -C 1 -C 6 -alkyl, -SO 2 -(C 1 -C 6)-haloalkyl, -SO 2 -aryl, -SO 2 -NR 10a R 10b, -P (OR 10a)(OR 10b), and C-linked amino acid or di-peptide, and R 10, R 10a, and R 10b are independently hydrogen or (C 1 -C 6)-alkyl optionally substituted with-OH, -NH 2, -NH(C 1 -C 4 alkyl), -N(C 1 -C 4 alkyl) 2, -C OH, -C O-(C 1 -C 4)-alkyl or halogen;