So the activity of many enzymes is basically controlled and regulated by creating some type of covalent bond, some type of covalent modification on that particular enzyme. And the most common type of modification that we create is the addition of a phosphoryl group by using an ATP molecule. So one example of covalent modification is the attachment of a phosphoryl group onto the enzyme. And as we discussed previously, when we discussed the kinase molecule, so NMP kinase inside our body, we have many different types of protein kinases which are used to basically catalyze the transfer of a phosphoryl group from our ATP molecule onto that particular enzyme. And by transferring that phosphoryl group, that can basically activate or inactivate the activity of that enzyme. Now the reverse process.