Structural domains comprising this superfamily share the structure of domain 1 of the cell-cell fusogen EFF/AFF. This three domain proteins are involved in cell fusion, as their name indicates, process that is essential for development. This proteins can be found either as a monomer or trimer, crystal structures of the trimer show unambiguous structural homology to class II viral fusion proteins in their characteristic postfusion hairpin conformation. The trimer subunits feature the three class II beta sandwich domains, termed I, II, and III, organized in the same way as in the viral proteins PMID:24725407.