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To assess the prevalence of Connexin 26 (GJB2), Connexin 30 (GJB6), and Pendred (SLC26A4) mutations in a population of adult cochlear implant patients with a history of either early idiopathic or hereditary progressive sensorineural deafness.Significant efforts have been applied in defining the epidemiology of Connexin 26 (GJB2)-associated hearing impairment in the pediatric population, yet the issue remains ambiguous for adult patients. Causation is important in this population because there are implications to prognosis, risk of associated medical manifestations, and for genetic counseling purposes.Adult patients meeting criteria for cochlear implantation with early-onset hearing loss assessed at an adult cochlear implant center from November 2007 to April 2009.Genomic DNA samples from whole blood were tested with bidirectional sequence analysis for mutations in the coding region of the GJB2 and SLC26A4 genes and tested for large deletions of the GJB6 gene.Fifty-seven patients were analyzed for GJB2 mutations. Eight patients (14%) were found to have GJB2-related hearing impairment; 5 patients were homozygous for the c.35delG mutation (genotype c.35delG/c.35delG), and 3 additional patients were compound heterozygotes with 2 different GJB2 mutations. Of these 8 patients with GJB2-related hearing impairment, 3 had serviceable hearing into their teenage years. One additional patient had 1 GJB2 variant (p.Met195Ile, heterozygous). None had GJB6 mutations.Of the 57 patients, 30 were also analyzed for SLC26A4 mutations. Three of these patients were compound heterozygotes for disease-causing SLC26A4 mutations, confirming SLC26A4-related hearing impairment. Three additional patients were found to have a single variant in SLC26A4.The prevalence of GJB2- and SLC26A4-related hearing impairment in an adult population with early-onset severe sensorineural hearing loss is significant, suggesting the need for routine assessment for genetic etiologies in this group. We also note 3 individuals with causal connexin 26 mutations with subjective serviceable hearing into adolescence in our cohort.