LIPID TRANSMEMBRANE DISTRIBUTION Methods used to determine lipid asymmetry None of the methods utilized is' perfect'. Each one has its own drawbacks and advantages. One must also keep in mind that the membrane which is studied may be perturbed with respect to its cellular state: for instance, evaluation of lipid distribution in an intracellular membrane requires cell lysis and isolation of the organelles. We will see that, in some cases, ATP plays a pivotal role in membrane asymmetry. Since during the isolation pro-cedure, the membrane is separated from cytoplasmicATP, a new lipid repartition may occur, different from that in situ. If one is interested in the plasma membrane of nucleated cells, one should add that membrane rupture during cell lysis induces an important perturbation of lipid transverse distribution (Connor et al., 1990; Schrier et al., 1992b).