A well established feature of apoptosis is the externalisation of the lipid phosphatidyl serine (PS) from the inner to the outer plasma membrane. Annexin-V (five) is a protein that specifically binds PS and fluorescent labelling of the annexin-V enables the flow cytometric detection of externalised PS, and hence apoptotic cells. When used in conjunction with a live/dead cell discriminator that measures membrane integrity (such as PI or 7AAD), early apoptotic cells (annexin-V positive only) can be distinguished from late apoptotic/necrotic cells (annexin-V and PI/7AAD positive). The early apoptotic phase can be quite rapid and can often be missed, making it appear that cells are either live or late apoptotic/necrotic. It is therefore often necessary to perform a time course experiment to prove that cells are in fact traversing through early apoptosis before reaching late apoptosis/necrosis.