Well, basically, when we discussed flip flopping, or also known as transverse diffusion, we said that proteins cannot actually rotate from one leaflet to the other leaflet, and that's because when they rotate, that is energetically unfavorable, because the proteins contain extensive hydrophilic. So polar regions, when they rotate, that means the polar regions must interact with the hydrophobic core of the membrane. And that is too energetically unfavorable. The energy is very high for that reaction and it doesn't actually take place. And so we see that proteins do not flip flop. They cannot rotate or move from one leaflet to the opposing leaflet.