MAPT and Alzheimer disease: Thus, future longitudinal studies in specific stages of AD may reveal a specific role for specific phospho-tau forms in the evolution of that stage e.g., p-tau231 and p-tau217 being early markers linking Aβ and tau pathology before tau PET becomes detectable, while in the AD dementia stage, decreases in tau phosphorylated at serine 202 and p-tau231 may reflect the ‘transition’ of soluble tau into insoluble tau in AD brain [22].