Learn how to use this product Yeast Viability Kit 1 is used to measure the cell concentration and viability of yeast samples with the LUNA™ family of cell counters for fluorescence cell counting. To assess the cell viability, Fluorescein Diacetate Stain and Propidium Iodide Stain for Yeast are used. Viable cells hydrolyze fluorescein diacetate into the green fluorescent compound fluorescein, causing viable cells to fluoresce green. Nonviable cells and cells with damaged membranes are the only ones that can take up Propidium Iodide (PI) since PI cannot penetrate intact membranes. When bound, the nuclei fluoresce red. Fluorescence Signal Enhancer 1 is an inhibitor of ATP-dependent fluorescein efflux. Fluorescence Signal Enhancer 1 lowers the background and increases signal intensity by blocking fluorescein from being pumped out of the cell, thereby enhancing the fluorescein signal-to-noise ratio. Fluorescence Signal Enhancer 1 is most effective with metabolically active cells in the log growth phase. Lastly, the Cell Dilution Buffer is for washing cells and diluting cell suspensions.