One of our main immunological findings is that two patients with anti-IgLON5 disease without brainstem tauopathy had an intense deposition of IgG4 in the neuropil and along the membrane of neurons in the tegmentum of the brainstem, olivary nuclei, and cerebellar cortex that co-localized with IgLON5. Here, IGLON5 is linked to tauopathy.