TK2 and mitochondrial DNA depletion syndrome: Increased lipid may be present in fibres with or without ragged red change and COX deficiency, e.g., in KSS and PEO due to mtDNA rearrangements [21], mtDNA depletion syndrome due to mutations in TK2, RRM2B, SUCLA2 and SUCLG1, and CoQ2 [31,56,123,124,125].