The causes of nonsyndromic hearing loss are complex. Researchers have identified more than 90 genes that, when altered, are associated with nonsyndromic hearing loss. Many of these genes are involved in the development and function of the inner ear. Mutations in these genes contribute to hearing loss by interfering with critical steps in processing sound. Different mutations in the same gene can be associated with different types of hearing loss, and some genes are associated with both syndromic and nonsyndromic forms. In many affected families, the factors contributing to hearing loss have not been identified.  Most cases of nonsyndromic hearing loss are inherited in an autosomal recessive pattern. About half of all severe-to-profound autosomal recessive nonsyndromic hearing loss results from mutations in the GJB2 gene; these cases are designated DFNB1. The GJB2 gene provides instructions for making a protein called connexin 26, which is a member of the connexin protein family. Mutations in another connexin gene, GJB6, can also cause DFNB1. The GJB6 gene provides instructions for making a protein called connexin 30. Connexin proteins form channels called gap junctions, which allow communication between neighboring cells, including cells in the inner ear. Mutations in the GJB2 or GJB6 gene alter their respective connexin proteins, which changes the structure of gap junctions and may affect the function or survival of cells that are needed for hearing.  The most common cause of moderate autosomal recessive nonsyndromic hearing loss is mutations in the STRC gene. These mutations cause a form of the condition known as DFNB16. Mutations in more than 60 other genes can also cause autosomal recessive nonsyndromic hearing loss. Many of these gene mutations have been found in one or a few families.  Nonsyndromic hearing loss can also be inherited in an autosomal dominant pattern. Mutations in at least 30 genes have been identified in people with autosomal dominant nonsyndromic hearing loss; mutations in some of these genes (including GJB2 and GJB6) can also cause autosomal recessive forms of the condition. Although no single gene is associated with a majority of autosomal dominant nonsyndromic hearing loss cases, mutations in a few genes, such as KCNQ4 and TECTA, are relatively common. Mutations in many of the other genes associated with autosomal dominant nonsyndromic hearing loss have been found in only one or a few families.  X-linked and mitochondrial forms of nonsyndromic hearing loss are rare. About half of all X-linked cases are caused by mutations in the POU3F4 gene. This form of the condition is designated DFNX2. Mutations in at least three other genes have also been identified in people with X-linked nonsyndromic hearing loss.  Mitochondrial forms of hearing loss result from changes in mitochondrial DNA (mtDNA). Mitochondria are structures within cells that convert the energy from food into a form that cells can use. Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. Only a few mutations in mtDNA have been associated with hearing loss, and their role in the condition is still being studied.  Mutations in some of the genes associated with nonsyndromic hearing loss can also cause syndromic forms of hearing loss, such as Usher syndrome (CDH23 and MYO7A, among others), Pendred syndrome (SLC26A4), Wolfram syndrome (WFS1), and Stickler syndrome (COL11A2). It is often unclear how mutations in the same gene can cause isolated hearing loss in some individuals and hearing loss with additional signs and symptoms in others.  In addition to genetic changes, hearing loss can result from environmental factors or a combination of genetic risk and a person's environmental exposures. Environmental causes of hearing loss include certain medications, specific infections before or after birth, and exposure to loud noise over an extended period. Age is also a major risk factor for hearing loss. Age-related hearing loss (presbyacusis) is thought to have both genetic and environmental influences.