The ApoAlert Annexin V-FITC Apoptosis Kit provides a simple and effective method to detect one of the earliest events in apoptosis—the externalization of phosphatidylserine (PS)—in living cells. Soon after apoptosis is induced, PS is translocated from the inner leaflet of the plasma membrane to the outer leaflet. This assay uses annexin V, which has a strong and specific affinity for PS, to monitor the PS translocation that occurs due to apoptosis. Labeled annexin V provides a simple staining assay to monitor apoptosis by flow cytometry or fluorescence microscopy.