Now, number four is actually what we spoke about previously. It's the fact that inside our cells we have these high energy adenosine triphosphate molecules that can act as the source of that phosphoryl group. So we see that phosphorylation requires the transfer of this phosphoryl group from some molecule onto that residue of that enzyme. And these high energy ATP molecules which are so abundant inside our cells can easily be used and very effectively be used to actually transfer that phosphoryl molecule. Number five is, and we'll discuss this in much more detail, we have the process of amplification that takes place whenever protein kinases are involved. And that's because usually a single protein kinase enzyme can actually catalyze many enzymes at once.