 I'm going to stay on the theme of signaling because this is fun, interesting in many ways. Another class of receptors is so-called RTKs. RTK. This is a receptor for a tyrosine kinase protein. Don't ask me about the order, but it's probably easier to think of the tyrosine kinase receptor. Receptor for tyrosine kinase proteins. They work in two kind of similar ways. So these are technically membrane proteins, but the part that goes through the membrane is frequently just one alpha helix. So it's almost just, it's not anchored in the membrane because it goes straight through the membrane, but the actual transmembrane part is very boring compared to all the other proteins we've looked at. Then you have a large part outside the membrane and a relatively large part inside the membrane. Normally these proteins exist as monomers, that is one unit, and then they're not active. Nothing happens. But when a molecule binds, there are two things that can happen. Either we have a molecule that here to the left, you have a, you see that the molecule that's red there and looks like cherries almost. In this case, both two monomers are required to bind the molecule. And when this molecule binds, the two molecules get together. I'll stay there for a second. Another alternative is that we have isolated molecules, single cherry in this picture. When that single cherry binds, it will somehow create each RTK monomer here to change its shape a bit. And when they change shape, suddenly they fall in love with each other and two of them will stick together. And then we're at the same page here for both of them. What then happens when these molecules are now stuck together, they will change their conformation first through the membrane and then on the part that is inside the membrane. When this part is on the side of the membrane changes, you get roughly the same process as with the GPCR, that there are connected proteins, in this case the kinase, and that kinase domain is then going through changes that cause a cascade inside the cell. They are super important for a lot of processes among them insulin. They're going to be related to all of the growth factors and everything, telling cells to divide. And that also means that if some things go wrong in these proteins, they can lead to very great side effects. I think this is going to be more fun if I show you a concrete example. So let's look at insulin. So insulin itself was a small globular protein, but the insulin receptor is gigantic. So here's we have the membrane, a small transmembrane helix, very large extracellular domain. We still don't know exactly what happens or how the signal to relax happens, but we now have structures of this protein with the insulin, the small orange or red molecule up there bound. And you might actually see here that the molecule has undergone through a slight conformational shift upstairs. And that means that down here in the basement on the inside of the cell, one way or another, these molecules are going to change the kinases here. This is the kinase and the active versus inactive structure. And there's a whole wealth of information here how this protein is changing its conformation. You've actually seen a movie of that, not specifically for the insulin receptor, but some of those Markov state models I've shown that everything have been used to simulate exactly how these proteins move in different conformations and how that creates a cellular cascade. I will come back to this when I talk about drug design. What can happen in some cases, not for the insulin receptor, but smaller and simpler R.D. case, they might have these alpha helices in the membrane so close to each other so that they pair up. And in that case, under some conditions, things go wrong, that the alpha helices end up having mutations in amino acids so that they stick together. And when they now stick together, they will keep signaling and telling the cell to divide. And then the cell will go berserk and create a tumor. So what if we could find a way to attack that, which people have tried to? But that's for the drug design part.