MAPT and Alzheimer disease: Proposed AD BBBs include the blood Aβ42/40, phosphorylated tau (p-tau) species—such as p-tau181, p-tau217, and p-tau231—as well as non-specific biomarkers like glial fibrillary acidic protein (GFAP) and neurofilament light chain (NfL) (Jack et al., 2024).