EACH MITOCHONDRION contains multiple copies of its own DNA (mtDNA), and possesses its own replication, transcription, and translation machineries.1 Each cell has thousands of copies of mtDNA molecules unlike the nuclear genome. Human mtDNA encodes 13 polypeptides of the respiratory chain (complexes I, III, IV, and V) subunits, 28 ribosomal RNAs, and 22 transfer RNAs (tRNAs), in a circular double-stranded genome of approximately 16.5 kilobases.2 Each of the 13 polypeptide-coding genes allows production of a single subunit of the respiratory chain, whereas the tRNA genes are involved in the synthesis of many respiratory chain components. While these mitochondrial gene products are used within the mitochondrion, they are insufficient to support the full function of the mitochondrion. A significant part of mitochondrial proteins are, indeed, coded by the nuclear genes. Furthermore, the endosymbiosis between mitochondria and the host cell is balanced on intricate interactions among genes and gene products coded by both nuclear and mitochondrial genomes. Thus, a defective mitochondrial protein(s), whether it is coded or regulated by the mtDNA or nuclear DNA, can influence mitochondrial functions.