MAPT and Alzheimer disease: In the proposed A/T/N classification system, the AD biomarkers are divided into three binary classes: “A” refers to the value of an Aβ biomarker (amyloid PET or CSF Aβ42); “T,” the value of a tau pathology biomarker (CSF p-tau or tau PET); and “N,” a quantitative or topographic biomarker of neurodegeneration or neuronal injury (CSF t-tau, FDG-PET, or structural MRI).