Apoptosis is characterized by a variety of morphological features. One of the earliest indications of apoptosis is the translocation of the membrane phospholipid phosphatidylserine (PS) from the inner to the outer leaflet of the plasma membrane. Once exposed to the extracellular environment, binding sites on PS become available for Annexin V, a 35-36 kDa, Ca 2+-dependent, phospholipid binding protein with a high affinity for PS. The translocation of PS precedes other apoptotic processes such as loss of plasma membrane integrity, DNA fragmentation, and chromatin condensation. As such, Annexin V can be conjugated to biotin or to a fluorochrome , and used for the easy, flow cytometric identification of cells in the early stages of apoptosis. Aplication Reference;