Mutations of mtDNA can be present as large-scale rearrangements (deletions or duplications) and point mutations. Small deletions and inversions have been described but these are rare and will not be considered further. Duplications are not considered pathogenic, although they may predispose to deletion formation. Many point mutations do not alter the corresponding protein sequence or have no demonstrable phenotypic effect but some do cause a defect of the respiratory chain. This is either through a direct effect on a structural subunit, or indirectly through an effect on intra-mitochondrial protein synthesis. Similar effects are seen from most large-scale deletions as they often affect both structural subunits and protein synthesis.
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