Exposure to aminoglycoside antibiotics such as gentamycin and tobramycin can lead to sensorineural, bilateral, and severe-to-profound hearing loss. A single base-pair substitution from an A to a G at position 1555 in the MT-RNR1 gene (encoding 12S ribosomal RNA) predisposes individuals to aminoglycoside ototoxicity. Evidence has shown that even a single dose of an aminoglycoside antibiotic results in irreversible hearing loss in Individuals with this mutation. The hearing loss is not dependent on aminoglycoside exposure. Approximately 40% of individuals with the A1555G mutation who have not been treated with aminoglycosides will develop hearing loss by 30 years of age, and the penetrance increases to 80% by age 65. The MTRNR1 gene is located within mitochondrial DNA, and the A1555G mutation is, therefore, transmitted by maternal inheritance. The mutation occurs as a homoplasmic change and has a prevalence of approximately 1% in the U.S. population.