The cerebrospinal fluid (CSF) α-synuclein seed amplification assay (αS-SAA) enables in vivo detection of pathogenic αSyn aggregates, but its clinical significance remains unclear.<h4>Methods</h4>We prospectively evaluated 108 individuals with mild cognitive impairment or mild dementia due to suspected AD undergoing lumbar puncture for anti-amyloid therapy (ATT) eligibility. The gene discussed is SNCA; the disease is Alzheimer disease.