Ponceau S, the sodium salt of a diazo dye, is a negatively charged, red colored stain commonly used for rapid and reversible protein staining and detection of protein bands on nitrocellulose or polyvinylidene fluoride membranes (PVDF) and cellulose acetate membranes. Stained protein bands appear as red bands on a pink background. This staining technique is often utilized to confirm protein electrotransfer in Western blotting process prior to antibody-based detection.