Propidium Iodide (PI) binds to double-stranded DNA and RNA after cells have been permeabilized. Once bound to nucleic acids, the nuclei appear red in color as observed by fluorescence microscopy with a rhodamine filter. PI is useful for multi-color fluorescent staining analysis. The bright red staining of nuclei makes this an excellent counter stain to be used simultaneously with FITC, PE, or Cy5. For analysis by flow cytometry, PI staining can be monitored in the FL2 channel. PI can also be used to measure cell viability since this dye is excluded from viable cells.