Mutations in mitochondrial DNA (mtDNA) have been found to be associated with nonsyndromic sensorineural hearing loss. Matrilineal relatives within and among families carrying certain pathogenic mitochondrial mutations exhibit a wide range of penetrance, severity, and age of onset of hearing loss, indicating that the mitochondrial mutations by themselves are not sufficient to produce a deafness phenotype. Modifier factors, such as nuclear and mitochondrial genes, or environmental factors, such as exposure to aminoglycosides, appear to modulate the phenotypic manifestations. [From OMIM]