Genetic disorders of chromosome X that manifest with hyperammonemia include Lowe syndrome (OMIM 309000, mutations in OCRL), Rett syndrome (OMIM 312750, mutations in MECP2), Barth syndrome (OMIM 302060, mutations in TAFAZZIN), ataxia (OMIM 312170, mutation in PDHA1), and hyperglycerolemia (OMIM 307030, mutation in GKD). The gene discussed is MECP2; the disease is atypical Rett syndrome.