SMPX and nonsyndromic deafness: To date, five X-linked genes have been found to be related to the etiology of non-syndromic hearing loss (NSHL): PRPS1 (DFNX1, OMIM:311850), POU3F4 (DFNX2, OMIM:300039), SMPX (DFNX4, OMIM:300226), AIFM1 (DFNX5, OMIM:300169) and COL4A6 (DFNX6, OMIM:303631).