Following technical and methodological improvements of the last decade, the underlying genes of a number of syndromes involving severe intellectual disability resembling Angelman and Rett syndromes have been identified. In order to keep track with these new entities, this special issue of Molecular Syndromology provides comprehensive reviews of the state of clinical and genetic knowledge about the 'old' entities of Angelman and Rett syndromes, as well as the newer syndromes related to MECP2 duplication or defects in the CDKL5, FOXG1, MEF2C, TCF4, NRXN1, CNTNAP2, SHANK3, EHMT1 and FOXP1 genes. Furthermore the special challenge that presents itself when seeking to establish a diagnosis in adult patients is discussed. All articles are authored by experts specializing in these particular syndromes. This publication should therefore provide a unique source of knowledge about these relatively common syndromes and should be an asset to all clinical geneticists, neuropediatricians, and researchers in the field of neurodevelopmental disorders. Since this collection of articles is presented within a single issue, it facilitates comparison between the different syndromes and seems to be destined to become a desk book that anyone involved in the field of medical genetics dare not overlook.