It should not come as too much of a surprise that biological membranes are considerably more complex than lipid bilayers. This has been made quite clear by the fluid-mosaic model which considers the cell membrane as a two-dimensional solution of a mosaic of integral membrane proteins and glycoproteins firmly embedded in a fluid lipid bilayer matrix. Such a model has several virtues, chief among which is that it allows membrane components to diffuse in the plane of the membrane and orient asymmetrically across the membrane. The model is also remarkable since it provokes the right sort of questions. Two such examples are Does membrane fluidity influence enzyme activity Does cholesterol regulate fluidity However, it does not go far enough. As it turns out, there is now another version of this model, the so-called post-fluid mosaic model which incorporates two concepts, namely the existence in the membrane of discrete domains in which specific lipid-lipid, lipid-protein and protein-protein