Schematic and rigidity results of HIV-1 Protease. The protein is a dimeric aspartyl protease (PDB file 1HHP)(left). The asymmetric unit (second from lefft) in the PDB file has one significant rigid cluster. Two of the asymmetric units make up the biological form of the protein. When the rigidity of the biological form of the protein is analyzed (second from right), the rigid clusters of the two individual monomers combine into one dominant rigid cluster. The black outlined region designates one of the two β hairpin loops often referred to as flaps, which function as chemical scissors and close in on the interior of the protein to facilitate an enzymatic reaction. The number of each type of rigid cluster (right) is listed for the asymmetric and biological units (AU = Asymmetric Unit, BU = Biological Unit).