Parkinson disease may be the proto-type for the alpha-synucleinopathies, but Alzheimer disease is in fact the most common form. While it is diagnosed by the presence of plagues and tangles in a demented individuals, over 50 percent of people with Alzheimer disease also have Lewy bodies. The Lewy bodies is considered the characteristic pathologic sign of Parkinson disease and contains abundant amount of alpha-synuclein. In addition, there is a well-recognized Lewy-body variant of dementia, Lewy bodies typically accumulate in Down syndrome, and they are common in familial Alzheimer disease, as well.