MAPT and Alzheimer disease: Fourth, previous studies have shown that t-Tau can be assayed in numerous peripheral organs, such as the submandibular gland, abdominal skin, scalp, colon, and liver in either AD or non-AD subjects [47]; these studies have also documented the decreased level of certain tau species, p-Tau231, in AD submandibular glands, which was inversely correlated with the Braak neurofibrillary tangle stage.