A molecular clamp is a polypeptide used to maintain the shape of proteins in some experimental vaccines. On a virus, pre-fusion proteins on their surface provide an attractive target for an immune reaction. However, if these proteins are removed or made by recombinant technology, they lose their shape and form what is called a "post-fusion form". When part of a virus, these proteins maintain their form by forming a quaternary structure with other viral proteins. The pre-fusion state of the protein is a higher energy metastable state. The extra energy is used to overcome the activation barrier of the fusion to the cell membrane. The virus protein (or part of it) in combination with the clamp polypeptide is called a chimeric polypeptide.