MAPT and Alzheimer disease: The recent cryoEM models of core PHFs extracted from AD brains may accommodate upstream of V306 an additional 16 amino acids, which could represent a mixture of residues 259–274 (R1) from 3R tau, or 290–305 (R2) from 4R tau [143,144], but in contrast, show 1:1 stoichiometry of R4 and R3 tau isoforms.