Glowing marks: A new class of protein stains, the pyrylium dyes, undergo a strong color change (typically from blue to red, see picture) on covalently binding to proteins. While the free stains are almost nonfluorescent, the protein-conjugated forms are highly fluorescent. The dyes do not alter the charge of a protein, and thus do not change its electrophoretic properties. The stains also can be used in quantitative protein assays.