MAPT and Alzheimer disease: The addition of a threefold stoichiometric excess of a second tau-based substrate, K19CFh (Online Resource Fig. 2b, d), slowed the AD-seeded reactions, but allowed a more dilute AD homogenate (10−5) to be discriminated from homogenates prepared from brains that were found to be free of tau pathology in immunohistochemical analyses (Online Resource Fig. 2b, d).