The hypothesis that the spread of tauopathy between neurons could be mediated by the passage of tau seeds across the extracellular space was inferred from in vivo studies [6, 19] and strengthened by in vitro studies using neurons that were engineered to develop tau pathology or stimulated to do so by the addition of exogenous tau seeds [5, 12, 68, 75]. Here, MAPT is linked to tauopathy.