When cells are undergoing cell death, phosphatidylserine(PS), a phospholipid which makes up a portion of the cell membrane, will translocate to the outside-facing side of the membrane. This translocation in the cell death mechanism exposes PSto the extracellular environment. Annexin V, a 35-36 kDa phospholipid-binding protein, has a high affinity for PS(kDa = 7 nmol/l). Once Annexin V is bound to the apoptotic cell it acts as a beacon for cells that have undergone apoptosis.
Apoptosis process can be measured in the laboratory easily and most effectively by using a FITC-labeled Annexin V protein to stain cells and analyze them via flow cytometry.