The known fusogenic proteins syncytin-1 and syncytin-2, together with the corresponding receptors ASCT2 and MFSD2A for syncytiotrophoblast fusion, are also linked to tumorigenic processes, whereby downregulation of syncytin-1 inhibits cell fusion between breast cancer cells and endothelial cells [35]. The gene discussed is ERVW-1; the disease is breast carcinoma.