There is a specific procedure for measuring the cell cycle using propidium iodide, there are 3 main steps. You must first fix the cells with ethanol, then add RNase, and then stain with the propidium iodide. Alcohols are better fixating agents than crosslinking fixatives, and that is the reason why ethanol is preferred to fix the cells. RNase is an important enzyme. In addition to staining DNA, PI also stains RNA, so that is the reason why cells must be treated with the RNase. However, when DAPI is used along with PI, RNase is not required since it already binds to RNA. The last step is simply staining with PI because the cells have been successfully fixed and ready to be stained.