SNCB and early-onset autosomal dominant Alzheimer disease: <h4>Introduction</h4>β-synuclein is a promising blood marker to track synaptic degeneration in Alzheimer's disease (AD) but changes in preclinical AD are unclear.<h4>Methods</h4>We investigated serum β-synuclein in 69 cognitively unimpaired mutation non-carriers, 78 cognitively unimpaired AD mutation carriers (asymptomatic AD), and 31 symptomatic mutation carriers from the Dominantly Inherited Alzheimer Network.<h4>Results</h4>β-synuclein levels were already higher in asymptomatic AD mutation carriers compared to non-carriers and highest in symptomatic carriers.