Even though for some peptidases the specificity has been clearly defined, in all probability only a few bonds will be susceptible to cleavage in a mature protein. A protein will have few bonds flexible enough to thread into a peptidase active site if the protein is in a native state, because of the stabilizing interactions within and between secondary structure elements within the substrate. It is widely assumed that the susceptible bonds will be within surface loops and interdomain connectors. However, once a bond is cleaved and the tertiary structure perturbed, further bonds may become susceptible.