Importantly, the degree of mtDNA depletion we see in MSTO1 patient fibroblasts (30–70% of the normal content) is consistent with the levels of mtDNA depletion reported in fibroblasts from OPA1 or MFN2 patients [41, 46, 49] as well as in mtDNA depletion syndromes (Fig. 6) [38]. This evidence concerns the gene OPA1 and mitochondrial DNA depletion syndrome.