Plasma NfL showed similarly good discrimination of pathology-confirmed AD from Aβ-PET–negative controls (p < 0.001, AUC 0.93 [0.87–0.99], optimal cutoff 45.7 pg/mL) but not from non-AD dementia cases (p = 0.33, AUC 0.68 [0.37–0.99]). This evidence concerns the gene NEFL and Alzheimer disease.