To assess if Env proteins encoded by HERV clades implicated in neurological diseases affect spreading of proteopathic seeds, donor HEK NM-HAagg cells were transfected with plasmids coding for epitope-tagged HERV-W Env Syncytin-1 or coding for an Env consensus sequence of the HERV-K HML-2 subgroup K113 (from now on termed HERV-K Env)41 (Fig. 7a, b). Here, ERVW-1 is linked to nervous system disorder.