Plasma phospho-tau 217 (pTau217) is a biomarker for Alzheimer’s disease (AD) pathology, reflecting amyloid (Aβ) and tau burden, but its role in Parkinson disease (PD) and 4-repeat(4R)-tauopathies remains incompletely understood. This evidence concerns the gene MAPT and early-onset autosomal dominant Alzheimer disease.