HPGDS and presbycusis: Moreover, a significant association between the NAT*6A polymorphism and age-related hearing loss has been reported: the genetic effect on presbycusis stemmed from the observation that NATs, together with cytochrome P450 and glutathione S-transferases, metabolize a wide range of xenobiotics and are important for the balance of oxidative status to protect cells against environmental toxins and the cellular damage caused by oxidative free radicals [34].