Hereditary hearing loss; about the known and the unknown.

Hereditary hearing loss is both clinically and genetically very heterogeneous. Despite the large number of genes that have been associated with the condition, many cases remain unexplained. Novel gene associations with hearing loss are to be expected but also are defects of regulatory regions of the genome which are currently not routinely addressed in molecular genetic testing and research. Inheritance patterns other than monogenic might be more common than assumed in isolated cases and diagnoses might have been missed because of misinterpretation of identified DNA variants. This review summarizes current insights in the genetics of hearing loss, the next steps that are being taken in research, and their challenges. Furthermore, genotype-phenotype correlations and modifying factors are discussed as these are instrumental in counselling hearing impaired individuals and/or their family members.