AzoDye-3 maleimide is an excellent building block for labeling thiol-containing molecules. 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, fluorescein derivatives such as FAM, HEX, TET, NED and JOE. This may reflect the AzoDye-3's ability to form stable, non-fluorescent complexes with a fluorophore.