Live or Dead™ Yeast CFDA-AM/Propidium Iodide Vitality Kit is a convenient way to test viability of yeasts. The kit uses CFDA-AM and Propidium Iodide. CFDA-AM is a cell permeable esterase substrate. The acetoxymethyl (AM) group of the substrate will allow CFDA to penetrate cell membranes. Once inside the cells, lipophilic blocking and diacetate groups are cleaved by esterases, resulting in green fluorescence. In contrast, Propidium Iodide only stain the yeasts with damaged membranes resulting in red fluorescence.