SNCB and early-onset autosomal dominant Alzheimer disease: METHODS: In this observational study, we investigated serum β-synuclein by immunoprecipitation-mass spectrometry (IP-MS) in 463 participants from the Alzheimer’s Disease Neuroimaging Initiative (ADNI) including clinically diagnosed cognitively unimpaired, mild cognitive impairment (MCI) and AD dementia subjects with ≥ 1 follow-up samples for 235 individuals and clinical follow-up for up to 19 years.