MAPT and Alzheimer disease: GSK3β is proline-directed serine/threonine kinase (i.e., SP/TP sites) that phosphorylates tau at more than 42 sites associated with AD [10], such as Tyr18, Thr181, Ser199, Ser202, Thr205, Thr217, Thr231, Ser396 and Ser422 [2].