Apoptotic cells lose their phospholipid membrane asymmetry, and this leads to exposure of phosphatidylserine (PS) at the cell surface. This process can be monitored using annexin V, which is a Ca2+-dependent phospholipid-binding protein with a high affinity for PS; further, annexin V is useful for identifying apoptotic cells with exposed PS.
By using FITC-labeled annexin V and propidium iodide (PI), it is possible to distinguish the stages of apoptosis. Permeability of the plasma membrane increases during the late stages of apoptosis, and therefore, PI can move across the cell membrane and bind to the cellular DNA. Therefore, the late-stage cells are stained with PI and annexin V, whereas early-stage cells are stained with only annexin V.