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Hearing loss is a very common congenital disorder affecting one in 1000 newborns. More than 50% of deafness cases in the paediatric population have a genetic cause with autosomal dominant, autosomal recessive, X-linked, or mitochondrial patterns of inheritance.1 Mutations in mitochondrial DNA (mtDNA), particularly in the 12S rRNA and tRNASer(UCN) genes, have been found to be one of the most important causes of sensorineural hearing loss (SNHL).2,3 The homoplasmic A1555G mutation in the highly conserved decoding site of the mitochondrial 12S rRNA has been found to be associated with both aminoglycoside-induced and non-syndromic SNHL in many families of different ethnic origins.4–8 Recently, the homoplasmic C1494T mutation in the same gene has also been found to be associated with aminoglycoside-induced and non-syndromic SNHL in a large Chinese family.9 In addition, a C-insertion or deletion at position 961 of the 12S rRNA gene has been shown to be associated only with aminoglycoside-induced deafness.10,11 Furthermore, the mitochondrial tRNASer(UCN) appears to be another hot spot for mutations associated with hearing impairment, as five deafness-associated mutations have been identified in the mitochondrial tRNASer(UCN) gene: A7445G,12,13 7472insC,14 T7510C,15 T7511C,16 and T7512C.17 However, non-syndromic deafness-associated mtDNA mutations, such as the A1555G4–8 or A7445G12,13 mutation, are often not sufficient to produce the clinical phenotype since some individuals carrying these mutations have normal hearing. Thus, other factors including other mtDNA mutations/polymorphisms and/or nuclear backgrounds or environmental factors modulate the phenotypic variability and penetrance of deafness associated with these mtDNA mutations. ### Key points