While A and T biomarkers reflect AD-specific pathology, N biomarkers—such as elevated CSF total tau (7), hypometabolism on 18F-fluorodeoxyglucose (FDG)-PET (8), and atrophy on structural MRI (9–11)—indicate neurodegeneration or neuronal injury from various etiologies and are therefore not specific to AD, representing a recognized limitation of the N category. This evidence concerns the gene MAPT and Alzheimer disease.