Reagent for detecting early stages of apoptosis. 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 biotin-conjugated Annexin V reagent can be visualized after binding to avidin-FITC or other tags.