This cleavage does not disassemble S1 from S2 in pre-fusion conditions (Belouzard et al., 2009), but the binding interaction of RBD with its receptor, accompanied by a further cleavage in a second site in S2 (S2’site, upstream of FP, Figure 4B), triggers the possible dissociation of S1 and the irreversible refolding of S2 into a “post-fusion” state (Figure 4B).