Usher syndrome is a clinically and genetically heterogeneous condition. Type IIIA is caused by a mutation in the CLRN1 gene (3q21-q25). It is inherited in an autosomal recessive pattern. Type IIIB (614505) is the result of homozygous mutations in HARS (5q31.3).
There is also a disorder resembling Usher that results from homozygous mutations in ABHD12 (20p11.21) that also causes PHARC (612674) (polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and early onset cataract).
This is the least common type of Usher syndrome. Three additional types of Usher syndrome are recognized: type I (276900) results from mutations in at least 7 different genes, type II (276901) from mutations in 4 genes, and Type IV resulting from mutations in the ARSG gene.