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In early apoptosis, cells lose membrane phospholipid asymmetry, whilst the integrity of the cell membrane is maintained. Phosphatidylserine (PS), a negatively charged phospholipid located in the inner surface of the plasma membrane, appears very early in apoptosis at the external cell surface. Annexin V, a calcium and phospholipid binding protein, binds preferentially to PS. The Annexin-V kits also contain a viability dye to evaluate membrane integrity. When cells are double-stained with annexin V and a viability dye, three different cell populations may be observed, live cells, necrotic or late apoptotic, and early apoptotic.