 I have to confess, I can't have lied to you. We don't have 20 amino acids. We have 19, because this one, proline, I will keep referring it to an amino acid, but strictly technically, purely technically, is an amino acid. The reason for that is that the side chain here has three carbons, one, two, three, but the last carbon here is actually binding to the nitrogen in the backbone. And what that will mean is that the normal amino acid side chains that we would have, they would have the hydrogen here, right? The hydrogen that can participate in hydrogen bonds in a helix. But in proline, we have this five-membered ring, one, two, three, four, five. It's stolen the hydrogen. There is no hydrogen, but instead, it's a large, unbaltered carbon. And that will mean that if it can't participate in those hydrogen bonds, and it will take a lot of room, it's going to be very unfavorable to put this, say, in a helix. In fact, proline is the best helix breaker we know. If I have a helix predicted, and I find a proline residue, I would say that that's going to be the end of that helix. And it's a very strong signal in bioinformatics. We can see that in the Ramachandran plot, too. So it's kind of just a line here, because this one of the five-psi torsions has been locked in, right? The five torsion. So there are only two small regions where it can be, and neither of them is really compatible with the alpha helix.