Subsequently, the RSK2 gene encoding a histone kinase was identified as the causative gene for Coffin-Lowry syndrome and non-specific XLMR [4,5], and the methyl-CpG-binding protein 2 (MeCP2) gene was identified as the causative gene for Rett syndrome [6] and other non-specific male MR [7-9]. Here, MECP2 is linked to Rett syndrome.