The Vybrant Apoptosis Assay Kit #4 detects apoptosis on the basis of changes that occur in the permeability of cell membranes. This kit contains ready-to-use solutions of both YO-PRO-1 and propidium iodide nucleic acid stains. YO-PRO-1 stain selectively passes through the plasma membranes of apoptotic cells and labels them with moderate green fluorescence. Necrotic cells are stained red-fluorescent with propidium iodide.