MAPT and Alzheimer disease: The development of AD models has allowed the individual study of tau fibrils and tau oligomers, which resulted in the conclusion that not only fibrillar [22, 25, 26, 44] but also oligomeric tau species can be bioactive [7, 13, 35, 43, 45, 53, 62, 66], but no detailed, direct comparison of these two species across multiple cases has been carried out.