The F homotrimer proteins that coat the membrane surface of RSV, start infection of host cells by promoting pH-independent fusion of cellular and virion membranes. In the cytoplasm the F homotrimers are synthesized from inactive precursor monomers of 574 amino acids. Each prefusion F monomer contains signal peptide, heptad-repeat A (HRA), heptad repeat B (HRB), transmembrane α-helix, and cytoplasmic tail (Figure ). Three monomers fold around a central top-to-bottom cavity, as shown by crystallographic 3D models (Figure ). The F homotrimers are then transported to the hostcell membrane driven by its signal peptides .