The bright and photo-stable 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, Ca²ᶧ -dependent affinity for PS and therefore serves as a probe for detecting apoptosis. The annexin V-EGFP fusion is brighter and photo-stable, ideal for detecting apoptosis by fluorescence microscopy with a FITC filter or by flow cytometry.