Annexin V conjugated to R-phycoerythrin (R-PE or PE) can be used to detect apoptotic cells by flow cytometry. The human anticoagulant Annexin V is a 35-36 kDa calcium-dependent phospholipid-binding protein with high affinity for phosphatidylserine (PS) . In normal viable cells, PS is located on the inner leaflet of the cytoplasmic membrane. In apoptotic cells, however, PS is translocated from the inner to the outer leaflet of the plasma membrane, where it can be detected by fluorescently labeled Annexin V.