MAPT and Alzheimer disease: The structural evidence that the Alzheimer’s disease fold involves the common parts of 3R tau and 4R tau may explain why PHF-like filaments induced by AD-tau seeds were observed not only in cells expressing both 3R tau and 4R tau, but also in cells expressing 3R tau or 4R tau individually (Fig. 6C, D and Supplementary Fig. 9, C).