Human endogenous retroviruses (HERVs) make up approximately 8% of the human genome. Although most HERVs are nonfunctional, the HERV-W (ERVWE1; MIM 604659) and HERV-FRD envelope (env) proteins can induce cell-cell fusion when expressed in cells possessing appropriate receptors (Blaise et al., 2003 [PubMed 14557543]).