Alpha-synuclein: One in a family of structurally related proteins that are prominently expressed in the central nervous system. Aggregated alpha-synuclein proteins form brain lesions that are hallmarks of some neurodegenerative diseases (synucleinopathies). The gene for alpha-synuclein, which is called SNCA, is on chromosome 4q21. One form of hereditary Parkinson disease is due to mutations in SNCA. Another form of hereditary Parkinson disease is due to a triplication of SNCA. See also: Parkinson disease gene.