Furin-like endoproteases form the host cell cleave F at two sites per monomer, producing p27 peptides , and generating disulfide-linked F (F1F2) prefusion homotrimers . The efficiency of furin-cleavage is Golgi-, cell line-, RSV subtype-and time-dependent. Therefore, uncleaved, partial or total cleavage F homotrimers may be present at the membranes of RSV virions and/or of infected host-cells . Additionally, prefusion homotrimers may exist as dynamic interchanges among ± partially linked p27 and/or monomer/trimer conformations 20,17, 18 . Most crystallographic F prefusion models, however, lack p27 because of its variable conformations and absence at the postfusion conformation .