This study is a 9-year natural history study for patients with Angelman syndrome in Belgium (a genetic neurodevelopmental disorder, affecting 500,000 individuals in the world). It includes a 3-year recruitment phase, a 5 year follow-up and a year to analyze the collected data. The investigators plan to include 30 patients with a semi-annual follow-up for 4 years. The investigators will collect relevant retrospective and prospective data using age-standardized scales and questionnaires for functional motor assessments and global developmental assessment.