Since the first discovery of MECP2 [194] and KDM5C [242] as X-linked genes that contribute to Rett Syndrome and Mental Retardation, X-linked, Syndromic, Claes-Jensen type (MRXSCJ) [196,243,244], further disease-causing mutations have been identified [245,246]. The gene discussed is MECP2; the disease is atypical Rett syndrome.