MAPT and Alzheimer disease: More recently, promising blood-based assays of these biomarkers have also been developed2–5, including sensitive and specific assays for the 42 and 40 amino acid-long amyloid β proteins (Aβ42/40 ratio, reduced levels of which reflect Aβ plaque pathology), phosphorylated tau (p-tau, reflecting AD-related tau phosphorylation) and neurofilament light chain (NfL, reflecting neuroaxonal injury irrespective of its cause).