The dysregulation of gene dosage due to duplication or haploinsufficiency involving dosage-sensitive genes is a major cause of multiple neurodegenerative diseases, such as the peripheral myelin protein 22 (PMP22) gene for Charcot-Marie-Tooth disease type 1A and the alpha-synuclein (SNCA) gene for Parkinson’s disease. This evidence concerns the gene SNCA and Parkinson disease.