Well, proteins have a much higher hydrophilic region, so polar region, than these fossil lipids do. And that's exactly why, if a protein is actually to flip flop, it has to overcome a very large energy barrier much higher than the energy barrier in the phospholipid rotation. And that's exactly why these proteins do not actually flip flop. They do not actually transverse that membrane. They only move along that lateral direction. So proteins, which have much more extensive hydrophilic polar regions, must overcome an even higher energy barrier.