Annexin V-Cy5 is a bright fluorescent reagent for detecting of the early stages of apoptosis. During apoptosis, phosphatidylserine (PS) is translocated from the cytoplasmic face of the plasma membrane to the cell surface. Annexin V has a strong, Ca²ᶧ-dependent affinity for PS and therefore serves as a probe for detecting apoptosis. Cy5 fluorescent dye produces an intense signal in the far-red region of the spectrum and therefore it is very useful for multiple labeling of cells with green and red colored fluorescent probes. Cy5 yields fluorescence with a λmax emission of 670 nm.