Ponceau S is a synthetic diazo dye primarily used in molecular biology and histology as a reversible protein stain. It is applied to visualize protein bands in polyacrylamide and agarose gels following electrophoresis, such as in SDS-PAGE and Western blotting. Unlike permanent stains, Ponceau S binds non-covalently to proteins, allowing easy destaining and preservation of protein integrity for subsequent analyses like immunodetection. It has a high affinity for protein molecules but is not approved or commonly used as a food colorant.