Well, we have an enzyme known as phosphorase kinase, which we'll discuss in just a moment. And what it does is it basically phosphorylates specific serine amino acids on this glycogen phosphorylase. So remember this is a dimer, we have two polypeptide chains, so one polypeptide chain and a second polypeptide chain. And the 14th amino acid position is a serene amino acid. So this one has a serine 14 and this one also has a seren 14. And phosphorase kinase can use two ATP molecules to actually attach the phosphoryl groups on the two seren 14 molecules to basically form phosphorase A.