PCSK1N and Alzheimer disease: Additionally, multiple cross-sectional studies have shown that α-syn seeding activity is associated with more severe motor dysfunction, cognitive impairment, brain atrophy, and reduced connectivity in PD, as well as worse clinical outcomes in Alzheimer's disease (AD).9, 10, 11, 12 As such, α-syn SAAs have had a remarkable impact on the field and have prompted calls for a biological definition and staging system of PD.13