And those proteins found outside the cells do not actually undergo this process of phosphorylation because outside the cells, ATP is not abundant. So reversible phosphorylation occurs inside the cells where the concentration of ATP is high and abundant. But proteins found outside the cells are not regulated in this method of phosphorylation because outside the cells we don't have an abundant supply of ATP like we have inside the cells. Now, the next topic that I'd like to discuss is why is phosphorylation so common? So why is this one of the more common methods of Covalent modifications? And as I mentioned earlier, it's because phosphorylation is very effective, very efficient and very convenient.