In Alzheimer’s disease and primary tauopathies progressive supranuclear palsy and cortico-basal degeneration, MRI studies indicate that tau accumulates in functionally connected brain regions.17-19 Using high-resolution array tomography imaging, we observe phosphorylated tau within synapses11,20 and even find pathological tau within synaptic pairs in brain regions affected late in the disease,21,22 supporting trans-synaptic spread of tau. Here, MAPT is linked to early-onset autosomal dominant Alzheimer disease.