This endogenous retroviral envelope protein has retained its original fusogenic properties (PubMed:15644441, PubMed:18077339, PubMed:22032925, PubMed:27589388). Together with Syna, participates in trophoblast fusion and the formation of a syncytium during placenta morphogenesis (PubMed:22032925). Synb is specifically involved in formation of syncytiotrophoblast layer II (SynT-II) (PubMed:22032925). Promotes myoblast fusion, and may play a role in regeneration of damaged muscle tissue in males (PubMed:27589388). May have immunosuppressive activity (PubMed:18077339). The mature protein consists of a trimer of SU-TM heterodimers (Probable). The SU-TM heterodimers are attached 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. Highly expressed in placenta where it localizes to syncytiotrophoblasts of the labyrinthine zona (PubMed:15644441). Specifically localizes to syncytiotrophoblast layer II (SynT-II) (PubMed:18448564). Also detected at very low levels in ovary (PubMed:15644441). Expressed in the placental labyrinth from stage 8.5 dpc onwards. In males, up-regulated in regenerating muscle tissue after injury. Synthesized as an inactive precursor that is heavily N-glycosylated and processed likely by furin in the Golgi to yield the mature SU and TM proteins. The cleavage site between SU and TM requires the minimal sequence [KR]-X-[KR]-R. 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. Isomerization of the intersubunit disulfide bond to an SU intrachain disulfide bond is thought to occur upon receptor recognition in order to allow membrane fusion. Viable, although growth is retarded and neonate numbers are reduced (PubMed:22032925). In the placental labyrinth, formation of syncytiotrophoblast layer II (SynT-II) is abnormal with reduced cell fusion and progressive expansion of maternal blood lacunae(PubMed:22032925). Unfused SynT-II cells form dense plaque-like structures between adjacent cells, which resemble cell junctions and are associated with high expression levels of GJB6/connexin-30 (PubMed:22032925). Formation of syncytiotrophoblast layer I (SynT-I) is grossly normal (PubMed:22032925). Adult male mice have a 15% reduction in muscle mass compared to wild type, probably due to defects in myoblast fusion (PubMed:27589388). Double knockouts of Syna and Synb are embryonic lethal at stage 9.5 dpc to 10.5 dpc, indicating a more severe phenotype than the Syna single knockout (PubMed:22032925). The mouse 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.