MAPT and Alzheimer disease: In this study, to identify the specific tau species responsible for propagation, we compared the uptake and propagation properties of different tau species derived from brain extracts of tau-transgenic mouse lines rTg4510 (expressing aggregating P301L tau (0N4R))19 and rTg21221 (expressing non-aggregating wild-type (WT) human tau (0N4R))20, human sporadic AD brain extracts, and recombinant WT full-length human tau (2N4R, 441 amino acid (aa)).