Schematic diagram of the functional domains of Env glycoproteins proposed to be involved in placental morphogenesis compared to a typical exogenous betaretroviral Env. ERV3 codes for a truncated Env in humans, with 1% of the population having a stop codon within the amino terminus of the SU region. Syncytin 2 (human) and syncytin-B (mouse) contain all the hallmarks of intact and functional Env proteins, including the leader peptide, furin cleavage site, fusion peptide, immunosuppressive domain, membrane-spanning domain, and a cytoplasmic tail.