MAPT and Alzheimer disease: The reason why tau protein is abnormally glycosylated in AD is unclear, but there are several hypotheses, including changes in the subcellular location of tau in AD leading to changes in its interaction with glycosylase, the increased activity of oligosaccharyltransferase (OST) in the dystrophic neurons in AD brain and the presence of a hitherto unidentified cytoplasmic N-glycosidase whose activity is down-regulated in AD brain due to certain factors [15].