PEX26 and Peroxisome biogenesis disorder-Zellweger syndrome spectrum: The genetic diagnosis was made before onset of RP in 10 young patients with apparently isolated hearing impairment: nine with Usher syndrome due to mutations in MYO7A and USH2A, and one with a peroxisome biogenesis disorder (PBD) due to compound heterozygous PEX26 mutations.