A variety of neurodegenerative disorders are classified as synucleinopathies based on the presence of prominent α-synuclein pathology. These diseases include Parkinson disease (PD) and dementia with Lewy bodies (with neuronal Lewy body formation) and multiple system atrophy (with glial cytoplasmic inclusions). The normal function of α-synuclein includes regulation of pre-synaptic vesicles. Autosomal dominant PD can be due to coding mutations or multiplications of the α-synuclein gene (SNCA). The coding mutations are thought to lead to a gain of function, in particular acceleration of the formation of proto-fibrils. Duplications and triplications of SNCA lead to autosomal dominant PD with a gene dosage effect on age of onset and clinical severity; variants in the SNCA promoter which lead to an upregulation of SNCA expression are associated with an increased risk of sporadic PD.