MAPT and Alzheimer disease: Mass analysis showed signals corresponding to tau peptides 392–406 (IVYKSPVVSGDTSPR), 388–395 (HGAEIVYK) and 375–385 (THKLTFRENAK) (Suppl Fig. 3b), strongly suggesting that the 7, 12 and 15 kDa AD tau bands represent tau fragments of 268–385, 268–395 and 268–406 derived from 3R tau isoforms, and 299–395, 299–385, and 299–406 derived from 4R tau isoforms (Fig. 4).