AzoDye-3 PEG 10 azide is an excellent building block for labeling alkyne-containing molecules. It contains a PEG10 spacer to increase its water solubility. AzoDye-3 dyes are nearly non-fluorescent with moderate extinction coefficients. These dark quenchers are most typically used in Molecular Beacon probes in which FRET has minimal contribution to their fluorescence quenching, thus dependence on donor-acceptor spectral overlap is not significant as to other FRET probes such as FRET peptides. AzoDye-3 dyes can quench a variety of fluorophores, in particular, Cy5 derivatives such as Cy5, Cy5.5, DyLight 647, Alexa Fluor® 647 and iFluor® 647. This may reflect the AzoDye-3's ability to form stable, non-fluorescent complexes with a fluorophore.