<unk>  I People with<unk> her I are born profoundly deaf and begin to lose their vision in the first decade of life. They also exhibit balance difficulties and learn to walk slowly as children, due to problems in their vestibular system.<unk>  type I can be caused by mutations in any one of several different genes: CDH23, MYO7A, PCDH15,<unk> H1C and<unk> H1G. These genes function in the development and maintenance of inner ear structures such as hair cells (stereocilia), which transmit sound and motion signals to the brain. Alterations in these genes can cause an inability to maintain balance (vestibular dysfunction) and hearing loss. The genes also play a role in the development and stability of the retina by influencing the structure and function of both the rod photoreceptor cells and supporting cells called the retinal pigmented epithelium. Mutations that affect the normal function of these genes can result in retinitis pigmentosa and resultant vision loss. Worldwide, the estimated prevalence of<unk>  type I is 3 to 6 per 100,000 people in the general population. Type I has been found to be more common in people of Ashkenazi Jewish ancestry (central and eastern European) and in the French-Acadian populations (Louisiana).<unk>  II People with<unk> her II are not born deaf and are generally hard-of-hearing rather than deaf, and their hearing does not degrade over time; moreover, they do not seem to have noticeable problems with balance. They also begin to lose their vision later (in the second decade of life) and may preserve some vision even into middle age.<unk>  type II may be caused by mutations in any of three different genes:<unk> H2A, GPR98 and DFNB31. The protein encoded by the<unk> H2A gene,<unk> herin, is located in the supportive tissue in the inner ear and retina.<unk> herin is critical for the proper development and maintenance of these structures, which may help explain its role in hearing and vision loss. The location and function of the other two proteins are not yet known.<unk>  type II occurs at least as frequently as type I, but because type II may be underdiagnosed or more difficult to detect, it could be up to three times as common as type I.<unk>  III People with<unk>  III are not born deaf but experience a progressive loss of hearing, and roughly half have balance difficulties. Mutations in only one gene, CLRN1, have been linked to<unk>  type III. CLRN1 encodes clarin-1, a protein important for the development and maintenance of the inner ear and retina. However, the protein's function in these structures, and how its mutation causes hearing and vision loss, is still poorly understood. The frequency of<unk>  type III is significant only in the Finnish population as well as the population of Birmingham, UK, and individuals of Ashkenazi Jewish heritage. It has been noted rarely in a few other ethnic groups.
Usher syndrome
Myhre syndrome
Sly syndrome
Irlen syndrome
McGillivray syndrome
Fabry disease
Bart syndrome
Sugarman syndrome