Parkinson's disease is the most common form of parkinsonism and is sometimes called "idiopathic parkinsonism", meaning parkinsonism with no identifiable cause.[1][26] Scientists sometimes refer to Parkinson’s disease as a type of neurodegenerative disease called synucleinopathy due to an abnormal accumulation of the protein alpha-synuclein in the brain.[27] The synucleinopathy classification distinguishes Parkinson's disease from other neurodegenerative diseases, such as Alzheimer's disease, where the brain accumulates a different protein known as the tau protein.[27]