The LIVE/DEAD™ FungaLight™ Yeast Viability Kit enables researchers to easily, reliably and quantitatively distinguish live and dead yeast in minutes by flow cytometry. The kit contains solutions of the SYTO 9 green-fluorescent nucleic acid stain and the red-fluorescent nucleic acid stain, propidium iodide. These stains differ both in their spectral characteristics and in their ability to penetrate healthy yeast cells. When used alone, the SYTO 9 stain generally labels all yeast in a population - those with intact membranes and those with damaged membranes. In contrast, propidium iodide penetrates only yeast with damaged membranes, causing a reduction in the SYTO 9 stain fluorescence by fluorescence resonance energy transfer (FRET) when both dyes are present. As a result, yeast with intact membranes stain fluorescent green, whereas yeast with damaged membranes stain fluorescent red.