MAPT and Alzheimer disease: Tau filaments from AD, prion protein amyloidoses, familial British dementia, familial Danish dementia and primary age‐related tauopathy are made almost entirely of R3, R4 and 10–12 amino acids after R4, all of which are sequences that 3R and 4R tau have in common [3, 8, 11, 12, 15].