Annexin V is used as a probe to detect cells that have expressed phosphatidylserine (PS) on the cell surface, an event found in apoptosis as well as other forms of cell death. The Annexin V affinity assay typically uses a conjugate of annexin V and a fluorescent or enzymatic label, biotin or other tags, or a radioelement, in a suitable buffer (annexin V binding to PS is Ca2+ dependent). The assay combines annexin V staining of PS membrane events with the staining of cell nucleus with PI or AAD-7 in living cells to distinguish from dead cells. Annexin V Apoptosis Detection Kit is based on the observation that soon after initiating apoptosis, cells translocate the membrane phosphatidylserine (PS) from the inner face of the plasma membrane to the cell surface. Once on the cell surface, PS can be easily detected by staining with a fluorescent conjugate of Annexin V, a protein that has a high affinity for PS. Detection can be analyzed by flow cytometry or by fluorescence microscopy.