MAPT and Alzheimer disease: We also noted that a unique phosphorylated Tau at Serine 396 (Tau pS396) in AD-affected neurons, targeted by active GSK-3β, was consistently detected in the culture medium of DS neurons on D45 (14.71 ± 3.71 pg/mL) but not in that of the control neurons at the same differentiation stage (Fig. 3d).