Center for Medical Genetics, Ghent University, Ghent University Hospital, Ghent, Belgium. Laboratory for Clinical and Experimental Neurophysiology, Neurobiology and Neuropsychology, Department of Neurology, Ghent University, Ghent University Hospital, Ghent, Belgium. Centrum voor Ontwikkelingsstoornissen (COS), Ghent University Hospital, Ghent, Belgium. Service de Génétique Médicale, Institut de Biologie, Nantes, France. Service de Pédiatrie, CHU, Nantes, France.