USH2A and Usher syndrome type 2: It is well known that genes have different prevalences in various countries and geographic areas, but most of the difference concerning the USH2A gene in our study can be explained by the fact that the patients with Usher syndrome type 2A are referred to us at an older age (mean age 39 at genotypic diagnosis in our registry) than the investigated group, since their visual decline becomes evident somewhat later in life.