The concept of lipid domains in membranes.

In the past two decades or so there has been considerable advance in our knowledge of membrane structure. The fluid mosaic model of Singer and Nicolson (1) has had great heuristic value in thinking about membrane topology. In this model both proteins and lipids are free to diffuse in the bilayer, implying a random organization of protein and lipid. However, experimental evidence in diverse types of membranes and for several protein entities indicates that the lateral motion of most proteins is not determined primarily by free diffusion through a two-dimensional viscous fluid, but is constrained, probably by mechanisms such as interactions with cytoskeletal components. We will here present our evidence that lipids may also have nonhomogeneity in their lateral distribution, i.e., the lipid may be organized in domains, and that such organizational