ERVW-1 and multiple sclerosis: In multiple sclerosis, a chronic inflammatory disease of the CNS in which the myelin sheath is damaged, the up-regulation of some HERVs and syncytin-1 protein was observed in patients’ brains, and the abnormal expression of syncytin-1 in astrocytes resulted in the induction of cytokines and reactive oxygen species, with ensuing oligodendrocyte damage [51].