Apoptosis(also called programmed cell death) is a kind of programmed and gene-controlled cell death type. It’s one of the key mechanisms for maintaining normal growth and homeostasis in organisms.

Annexin V is the indicator for detecting early cell apoptosis, which has a high affinity for PS(photosensitive serine). In the early apoptosis, PS flips from the inner side of the lipid bilayer of the cell membrane to the outer side. Recombinant Annexin V coupled with fluorescein was used to detect PS on the surface of cell membranes. In the late apoptosis, PI(Propicium Iodide) can stain the nucleus red(except normal cells) after entering the cell membrane. So cells at different stages of apoptosis can be grouped by staining with the combianant of Annexin V labeled FITC/EGFP and PI.