Introduction
For more information on human mitochondrial DNA, consult MITOMAP, a human mitochondrial genome database. Hearing loss often occurs as an additional symptom in a number of syndromic diseases caused by mitochondrial DNA defects. It is common for mitochondrial mutations  to give rise to different phenotypes in different families or patients. The most striking example of this pleiotrophy is the 3243A->G mutation, that can give rise to different conditions including MELAS, MIDD, MERFF and PEO. Nonsyndromic hearing loss also can be caused by mitochondrial mutations, as demonstrated by the finding of several mtDNA mutations in families with maternally inherited hearing loss that is nonsyndromic in many patients.