Annexin V is a protein that binds to phospholipids, particularly phosphatidylserine, which is typically found on the inner leaflet of cell membranes. During processes like apoptosis (programmed cell death), phosphatidylserine is exposed on the outer membrane, making Annexin V a useful marker for detecting early stages of cell death. In laboratory settings, Annexin V is often used in flow cytometry and immunofluorescence assays to identify apoptotic cells. By tagging Annexin V with a fluorescent dye, researchers can visualize and quantify cells undergoing apoptosis, aiding in studies related to cancer, neurodegenerative diseases, and other conditions where cell death plays a critical role.