Although one of the first studies in patient fibroblasts to examine mtDNA in the context of MFN2 variants (T105M, I213T, V273G and L734V) did not report changes to mtDNA copy number, nor the presence of deletions (Amiott et al., 2008), subsequent work noted the accumulation of mtDNA deletions in patient fibroblasts with the D210V variant, which was linked to an additional optic atrophy phenotype (Rouzier et al., 2012). The gene discussed is MFN2; the disease is optic atrophy.