phospholipid redistribution estimation In a viable cell, the phospholipid, phosphatidyl serine (PS) is localized on the inner leaflet of the cell membrane facing the cytosol. When the cell is undergoing apoptosis, the cell membrane loses its phospholipid asymmetry and exposes PS on the outer layer of the membrane. Annexin V specifically binds PS. Hence, apoptotic cells can be detected by staining with FITC conjugated Annexin V followed by flow cytometry. viability measurement method