The disorder splits into three clinical categories (Types I–III), relating to severity and age of onset28,30 and homozygous USH2A mutations result in the majority of Type-II cases in which diagnosed individuals are born with hearing loss and develop retinitis pigmentosa at the onset of puberty, yet do not experience the vestibular dysfunction (i.e. difficulties with balance and coordination) found in other types of Usher31. Here, USH2A is linked to hearing loss disorder.