But the one that we're going to focus on is known as phosphorylation. And phosphorylation is a very common method of covalent modification. It's a very common method by which we actually control the activity of enzymes and change the functionality of proteins. In fact, it's so common that nearly 30% of all the proteins and this includes enzymes in eukaryotic cells are actually phosphorylated. Now why is this method so common? Well as it turns out phosphorylation is a very effective, very efficient and a very convenient process and we'll see why that is.