It is not absolute because these lipids can actually rotate, can actually flip flop from one leaflet to the other. So we see that however, lipids such as fossil lipids can actually flip flop and therefore the asymmetry of the fossil lipids in that membrane does not remain absolute, it changes over time. However, we know that flip flopping of fossil lipids actually takes place very, very slowly because it is also energetically unfavorable. In fact, certain lipids, such as Glycolipids that contain very large components that are polar, cannot actually rotate because of those polar interactions, because these polar interactions will be unstabilized when they interact with that core hydrophobic region. And so, just like proteins, glycolipids cannot actually flip flop or rotate. Now, nevertheless, we see that lipids such as glycolipids and spingolipids can and do lead to asymmetry of the membrane.