The SensoLyte 520 cathepsin D activity assay kit from AnaSpec Inc. incorporates a fluorescence resonance energy transfer substrate that fluoresces at a much higher wavelength than standard ones. The higher wavelength fluorescence results in cathepsin detection that avoids autofluorescence interference from cell components and test compounds. The kit provides enough reagents to perform 100 assays in a 96-well microplate format. It includes the following components: 5-FAM/QXL 520, cathepsin D substrate, and 5-FAM, fluorescence reference standard, both Ex/Em = 490 nm/520 nm; cathepsin D, bovine spleen; assay buffer; pepstatin A; and DTT. Cathepsin D is the lysosomal aspartic proteinase implicated in intracellular protein degradation.