Cerebrospinal fluid levels of P-tau mirror changes in the brain tau metabolism and are elevated in all disease stages of AD, including the asymptomatic phase when tau-PET is still normal.13,14,15 Tau-PET tracers bind to insoluble paired helical filaments of tau in neurofibrillary tangles, and tau-PET measures become abnormal mainly in symptomatic AD and correlate with brain atrophy and cognitive function.16,17 Together, these findings suggest that fluid-based measurements of P-tau may be more sensitive than tau-PET in the earliest stages of AD. This evidence concerns the gene MAPT and Alzheimer disease.