In the prefusion state the fusion proteins exist in a stable conformation on the surface of the virus. After a triggering step, which varies among viruses, the extended intermediate is formed. This transient conformation collapses to form the final post-fusion structure where lipid mixing can occur. First, in hemifusion, only the outer membranes have mixed. Finally, when the fusion pore is formed, the inner membranes can mix, and the virus can enter the cell.