Consistent with the autopsy findings that the accumulation of tau inclusion in AD propagates in a stereotypical pattern along neuroanatomically interconnected brain regions [159], multiple lines of evidence indicate that pathogenic misfolded tau proteins are secreted into the extracellular space with the ability to transfer from affected neurons to neighboring healthy neurons and to promote misfolding and aggregation of normal tau molecules in healthy neurons that have internalized pathogenic tau [160,161]. This evidence concerns the gene MAPT and Alzheimer disease.