MAPT and Alzheimer disease: MALDI-TOF MS analysis of trypsin-resistant tau from AD brains without alkaline phosphatase treatment gave signals corresponding to molecular masses of 10,517.1 (3) and 9403.2 (5), which match the calculated values of residues 268–395 and 268–385 of human 3R tau isoforms, and molecular masses of 10,447.2 (4) and 9330.1 (6) which match the calculated values of residues 299–395 and 299–385 of human 4R tau isoforms (Suppl Fig. 3a).