This endogenous retroviral envelope protein has retained its original fusogenic properties and participates in trophoblast fusion and the formation of a syncytium during placenta morphogenesis. The interaction with MFSD2A is apparently important for this process (By similarity). Endogenous envelope proteins may have kept, lost or modified their original function during evolution and this one is unable to confer infectivity. The surface and transmembrane proteins form a heterodimer. They are attached by non-covalent interactions or by a labile interchain disulfide bond (By similarity). The surface protein is not anchored to the membrane, but localizes to the extracellular surface through its binding to TM. The CKS-17 immunosuppressive domain is present in many retroviral envelope proteins. As a synthetic peptide, it inhibits immune function in vitro and in vivo (By similarity). Specific enzymatic cleavages in vivo yield the mature SU and TM proteins. The CXXC motif is highly conserved across a broad range of retroviral envelope proteins. It is thought to participate in the formation of a labile disulfide bond possibly with the CX6CC motif present in the transmembrane protein (By similarity). In contrast with its primate orthologs, this envelope does not confer infectivity to pseudotyped virions. This is due to a N to T transition at position 532 in the cytoplasmic tail of the TM protein. Ortholog of the human HERV-FRD_6p24.1 envelope protein. The 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. Belongs to the gamma type-C retroviral envelope protein family. HERV class-I FRD env subfamily. CKS-17 sequence does not match the minimal active consensus.