Hereditary hearing loss is a complex disorder that involves a large number of genes. in developed countries, I in 1,000 children is born with deafness severe enough to require special education services, and about 60% of the cases of isolated deafness have a genetic origin. Although more than 100 genes for hearing loss are known currently, only a few are routinely tested in the clinical practice. In this study, we present our findings from the molecular diagnostic screening of the GJB2 and GJB3 genes, del(GJB6-D13S1830) and del(GJB6-D13S1854) deletions in the GJB6 gene, Q829X mutation in the otoferlin gene (OTOF) and, the A1555G and A7445G mutations in the mitochondrial genome over an 8-year period. Mutations analysis in the previously mentioned genes and Mutations was performed on 645 unrelated Brazilian patients with hearing loss who fell into two different testing groups. Different mutations in the GJB2 gene were responsible for most of cases studied, but deletions in the GJB6 gene as well as mitochondrial mutations were also found. While most cases of hearing loss in this country are due to environmental factors, the genetic etiology of deafness will increasingly be determined as more genetic tests become available. (C) 2007 Wiley-Liss, Inc.143A1415741579