Annexin V is a member of the annexin family, which binds to phosphatidylserine (PS) in a calcium-dependent manner. The Annexin V-labeled fluorescein bind specifically to the PS on the outer leaflet apoptotic cell membrane and can be detected with flow cytometry or fluorescence microscope. Nuclear dyes (PI, DAPI, 7-AAD) have a high DNA binding constant and is efficiently excluded by intact cells. It is useful for DNA analysis and dead cell discrimination during flow cytometric analysis. Due to late apoptosis or loss of membrane integrity in necrotic cells, nuclear dyes can enter cells to stain DNA, and when used in combination with Annexin V, cells in different apoptotic stages can be distinguished.