MAPT and Alzheimer disease: We know that hallmark biomarkers used to classify the amyloid, tau and neurodegeneration (ATN) profile in plasma, including amyloid peptides, several P-Tau isoforms and neurofilament [119], are elevated in both AD and DS–AD [20,120,121], but there has been no systematic study that quantifies the other isoforms of Tau in this unique niche.