May play a role in trophoblasts syncytialization, the spontaneous fusion of their plasma membranes, an essential process in placental development. May negatively regulate cell-cell fusion by interacting with SLC1A5, the probable receptor on the cell surface of the fusogenic syncytin-1/ERVW-1. Interacts (secreted) with SLC1A5; mainly at cell surface. Also found in cell lysates. Specifically expressed in placenta by extravillous trophoblasts and syncytiotrophoblasts (at protein level). The human genome contains a high percentage of proviral-like elements, also called endogenous retroviruses (ERVs) that are the genomic traces of ancient infections of the germline by exogenous retroviruses. Although most of these elements are defective, some have conserved a functional envelope (env) gene, most probably diverted by the host for its benefit.