Annexin V is a calcium-dependent phospholipid-binding protein that has been widely used as a molecular probe for detecting and quantifying apoptotic cells. Annexin V has a high affinity for phosphatidylserine, a phospholipid that is normally located on the inner leaflet of the plasma membrane but becomes exposed on the outer surface of the membrane during the early stages of apoptosis. By binding to phosphatidylserine, Annexin V can be conjugated with fluorophores or other detection molecules to label apoptotic cells for flow cytometry, fluorescence microscopy, or other imaging techniques. The use of Annexin V in apoptosis assays has become a standard method for studying cell death processes in research and clinical settings.