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Parkinsonism in GNDs312IntroductionWhile genetic brain disorders are traditionally dichotomized into neurodevelopmental and neurodegenerative disorders, it is becoming clear that some conditions are associated with both neurodevelopmental problems and neurodegeneration,1 and there are indications for a shared underlying genetic susceptibility.2, 3 Indeed, with advances in clinical genetic testing for neurological disease and an increase in life expectancy due to improved medical care, a growing number of genetic neurodevelopmental disorders (GNDs) has been associated with the development of Parkinson’s disease and other forms of parkinsonism.4-6Although individually rare, research in GNDs may help to better understand different pathophysiological mechanisms that are believed to play a role in the development of parkinsonism. An analogy may be found in Down syndrome that is associated with early-onset Alzheimer’s dementia. Research in Down syndrome has provided important insights into dementia etiology,7 and has facilitated research into disease-modifying treatments.8, 9 Genetic variants associated with GNDs are often identified in childhood, before neurologic symptoms emerge, and long-term follow up of children and adolescents with GNDs may increase knowledge on disease trajectories. In addition to the potential to identify disease and/or mechanism-specific treatment through animal models that are available for many genetic conditions, recognition of GNDs associated with parkinsonism may improve anticipatory care for patients with these GNDs. Thus, knowledge about GNDs that may present with parkinsonism is important to optimize clinical practice and further research.In this systematic literature review, a comprehensive overview is provided of studies that reported on parkinsonism in GNDs. We summarize findings related to patient characteristics, parkinsonian features and proposed disease mechanisms, and outline implications for clinical practice and future research.