The Fluorescein Diacetate (FDA) and Propidium Iodide (PI) assay is used for yeast cell viability testing. FDA is a cell viability probe that is membrane-permeable to live yeast cells. Inside a live cell, FDA is cleaved to give fluorescein by the action of intracellular esterases. Propidium Iodide dye for viability assessment. PI is a nucleic acid-binding dye that is impermeable to live cells and suitable for staining dead, nucleated cells. Live cells are reported by FDA in the green channel, dead cells by PI in the red channel.