During apoptosis, phosphatidylserine (PS) is translocated from the cytoplasmic face of the plasma membrane to the cell surface. Annexin V has a strong, Ca2 -dependent affinity for PS and therefore is used as a probe for detecting apoptosis. The one-step staining procedure takes only 10 minutes. In addition, the assay can be directly performed on live cells. Annexin V-PE-Cy5 stained cells can be detected by flow cytometry or fluorescence microscopy.