The SensoLyte® 520 TACE (tumor necrosis factor alpha converting enzyme) assay kit offers fluorescence detection at long wavelengths. This kit shows less autofluorescence interference from test compounds and cellular components, and helps provide better assay sensitivity than comparable kits using shorter-wavelength FRET substrates. The SensoLyte 520 TACE activity assay kit uses a FRET peptide substrate for the continuous measurement of enzyme activity. The FRET pair is composed of 5-FAM (5-carboxyfluorescein) and QXL™ 520. In the intact FRET peptide, the fluorescence of 5-FAM is quenched by QXL™ 520. Upon cleavage of the FRET peptide by the active enzyme, the fluorescence of 5-FAM is recovered, and can be continuously monitored at excitation/emission = 490 nm/520 nm. The kit can be used to detect the activity of TACE or screen for TACE inhibitors.