And what that means is inside the active side of kymatryptin, we have some type of nucleophilic residue that acts as the catalyst in forming a covalent bond, a temporary covalent bond between the active side of the enzyme and that peptide substrate molecule. Now, we say that the covalent bond is temporary because at the end of the reaction, that bond is broken. And that's important because we have to reform that unmodified enzyme, we have to regenerate that enzyme. At the end of any enzyme, catalyzed react. Remember, enzymes are never changed or depleted at the end of the reaction. They always have to be regenerated.