In this connection, the co-cultivation of human prostate cancer cells with muscle cells resulted in an IL-4 and IL-13 dependent upregulation of syncytin-1 and annexin 5 expression and enhanced frequency of homotypic cancer cell–cell fusion [98] suggesting that both cytokines might be involved in the conversion towards a pro-fusogenic state of (cancer) cells. Here, ERVW-1 is linked to cancer.