Chromosome X is enriched in genes associated with neurological symptoms, specifically, intellectual disability [50,51], including fragile X syndrome (OMIM 300624, mutation in FMR1), Lesch–Nyhan syndrome (OMIM 300322, mutation in HPRT), Hunter syndrome (OMIM 309900, mutation in IDS), lissencephaly (OMIM 300067, mutation in DCX), McLeod syndrome, and Norrie disease (OMIM 310600, mutation in NDP). Here, NDP is linked to fragile X syndrome.