USH2A and Usher syndrome: Molecular diagnosis in Usher syndrome is hindered by significant genetic heterogeneity, the large size of some of the Usher genes, and the high number of polymorphic variations in genes such as MYO7A and USH2A. Furthermore, a digenic inheritance has been proposed in some cases of Usher syndrome [10], although not yet universally accepted [13], which further complicates the elucidation of the molecular basis of genotype-phenotype correlation.