Another characteristic of apoptotic cells that can be exploited in this death quest is the appearance of phosphatidylserine (PS) on the outer surface of the membrane. Annexin V binds with high affinity to PS and can be used to spotlight dying cells. Fourteen companies offer Annexin V with a variety of tags and detectors attached that can be used immunohistochemically or with the flow cytometer (Alexis, BioWhittaker, Boehringer Mannheim, Chemicon, CLONTECH, Immunotech, Genzyme Diagnostics, Kamiya Biomedical, Oncogene Research Products, Oncor, Pharmingen, R & D Systems, Trevigen, Zymed). Some kits come supplied with reagents to differentiate necrotic cells from apoptotic cells, relying on the fact that the plasma membrane of apoptotic cells, flipping notwithstanding, is still intact. Thus certain dyes taken up by necrotic cells are excluded from apoptotic cells. These reagents have the advantage of being usable on intact, even live, cells; they can be used quantitatively; and they generally...

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