Mutations in the gene for connexin 26, GJB2, are the most common cause of hearing loss in American and European populations, with a carrier rate of about 3%-a rate similar to that for cystic fibrosis. A single mutation, 35delG, is responsible for most of this autosomal recessive hearing loss, DFNB1. A broad spectrum of mutations in GJB2 has been found to be associated with hearing loss, including another deletion mutation, 167delT, which has a carrier rate of about 4% in the Ashkenazi Jewish population. Mutations in GJB2 have also been found to be associated with dominant nonsyndromic hearing loss, DFNA3. Clinical studies have shown that the recessive hearing loss can vary from mild to profound, even within the same sibship. This type of hearing loss is nonsyndromic and is accompanied by normal vision, vestibular responses, and no malformations of the inner ear detectable by computed tomography scanning. Progressive and asymmetrical hearing loss has been noted in some cases, but it accounts for fewer than one-third of the cases of this type of hearing loss. The discovery of mutations in GJB2 that cause hearing loss has profound implications in the early diagnosis of hearing loss in general. The relative ease of diagnosis by genetic testing of Cx26 permits early identification of children with GJB2/DFNB1 hearing loss. This testing, coupled with hearing loss diagnosed by infant auditory brainstem response audiometry, will ensure that hearing-impaired children and their parents receive proper medical