HERV-W env, encoded from a full-length provirus at locus 7q21.2, gives rise to a syncytin glycoprotein (Blond et al., 2000; Mi et al., 2000), the expression of which, similar to HERV-W env, is increased by threefold in the brain tissue of MS patients compared with the controls (Antony et al., 2004; van Horssen et al., 2016). Here, ERVW-1 is linked to myeloid sarcoma.