QXL™ 670 dyes are optimized quenchers for Cy5 and Cy5-like fluorophores such as HiLyte Fluor™ 647. Their absorption spectra perfectly overlap with the fluorescence spectra of Cy5-like dyes. This characteristic makes QXL™ 670 dyes excellent quenchers for Cy5-like fluorophores. QXL™ 670 acid, SE readily reacts with amino-containing compounds.