Human mitochondrial disorders comprise a heterogeneous group of multisystem diseases, characterized by morphological, biochemical, or genetic abnormalities of mitochondria. Mutations in mtDNA have been described predominantly in a variety of rare encephalomyopathies but are also emerging in association with more common disorders, such as sensorineural hearing loss (SNHL) and cardiomyopathies (DiMauro and Bonilla 1997). Most of the identified mtDNA mutations are associated with specific clinical phenotypes (DiMauro and Bonilla 1997). In a recent issue of the Journal, Estivill et al.(1998) reported that the A1555G mutation in the mitochondrial 12S rRNA is responsible for a significant number of cases of maternally inherited nonsyndromic hearing loss and that its pathogenic role is enhanced by treatment with aminoglycosides. Idiopathic cardiomyopathies are an important cause of morbidity and mortality throughout the world, both in children and adults, with an annual incidence of 2–8/100,000 in the United States and Europe (Manolio et al. 1992). The application of molecular genetic techniques has started to delineate the molecular bases of these syndromes through the demonstration of alterations of myocardial contractile and structural proteins, such as the cardiac b-myosin heavy-chain (MYH7) gene, which accounts for∼ 75% of the familial cases of hypertrophic cardiomyopathies (Geisterfer-Lowrance et al. 1990). There is growing evidence that mtDNA mutations can cause cardiac disease, including cardiomyopathies and cardiac conduction block. In addition, cardiomyopathy may result from bioenergetic defects caused by mutations in nuclear-encoded subunits of the respiratory chain or in nuclear genes controlling the integrity, replication, and expression of mtDNA (Cortopassi et al. 1992; Kelly and Strauss 1994; DiMauro and Bonilla 1997).