A protein best known for its role in Parkinson's disease may help explain why women make up around two-thirds of Alzheimer's cases , according to new research. Researchers from the Mayo Clinic in the US studied a group of 415 individuals, including cognitively healthy people and those with mild cognitive impairment or dementia, to identify biomarkers of disease. In people who had abnormal alpha-synuclein in their cerebrospinal fluid and signs of Alzheimer's-related brain changes on PET scans ,