Mutations in the SLC25A4 gene, encoding AAC1, one of the four human AACs [71], have been reported to cause three different diseases: adult-onset autosomal-dominant progressive external ophthalmoplegia 2 (AdPEO2) [72] and recessive and dominant early-onset mitochondrial myopathy/cardiomyopathy assigned as mitochondrial DNA depletion syndrome (MTDPS) type 12B [73,74,75,76] and 12A, respectively [77] (Table 1). The gene discussed is SLC25A4; the disease is mitochondrial DNA depletion syndrome.