AzoDye-1 PEG10 Succinimidyl Ester is an excellent building block for labeling amino-containing molecules. It contains a PEG10 spacer that significantly improves its water solubility compared to AzoDye-1 succinimidyl ester (#2402) that bears the same extremely hydrophobic BHQ-1 chromophore. AzoDye-1 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-1 dyes can quench a variety of fluorophores, in particular, fluorescein derivatives such as FAM, HEX, TET, NED and JOE. This may reflect the AzoDye-1's ability to form stable, non-fluorescent complexes with a fluorophore.