Akin to HERV-W Env-derived Syncytin mediation of cell fusion in the placenta, HERV-W protein upregulation in breast and endometrial cancers has been associated with increased cell-cell fusion, occurring with cancer progression, metastasis, and chemoresistance (Fig. 3b) [5, 154, 155]. The gene discussed is ERVW-1; the disease is endometrial cancer.