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Chapter 232MethodsThe study protocol was published in the PROSPERO International Register for Systematic Reviews (CRD42020191035). We made a few minor amendments to the initial protocol, including the addition of two co-authors, a repeated search, and inclusion of non-rare GNDs (e.g., Down syndrome). We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Protocol (PRISMA).10Search strategy and selectionWe performed a comprehensive literature search in PubMed and Embase on June 15, 2021 (see the Supporting Information Supplementary Methods and Supplementary Figure S1). We used the Human Phenotype Ontology (HPO; https://hpo.jax.org/) term “neurodevelopmental abnormality” (HP:0012759) for GNDs, in combination with database-specific subheadings and text words for “Parkinson’s disease/parkinsonism”.11 Two reviewers independently screened titles and abstracts for eligibility. Relevant records were screened against selection criteria for inclusion based on full-text. In case of uncertainty, two other reviewers were consulted. Discrepancies were discussed until consensus was reached.We included all reports on patients who met the inclusion criteria for a neurodevelopmental disorder (i.e., listed in the HPO as “neurodevelopmental” and/or representing patients showing a deviation from normal of the neurological development in childhood, including any or all aspects of the development of personal, social, motor, and cognitive abilities, in the presence of a disease-causing genetic variant) and parkinsonism (bradykinesia in combination with either rigidity, rest tremor or both),12 or who were likely to have Parkinson’s disease, operationalized as a clear beneficial response to levodopa, reduced dopamine transporter binding with dopaminergic imaging and/or neuropathological hallmarks of Parkinson’s disease. We considered hypokinesia, akinesia, and hypomimia equal to bradykinesia if parkinsonism was diagnosed, in case bradykinesia was not explicitly noted. We excluded: 1) reports on patients with uncertain genetic etiology; i.e., not molecularly confirmed or according to standard clinical diagnostic criteria, 2) ultra-rare genetic conditions with less than