Cathepsins are a class of globular lysosomal proteases, playing a vital role in mammalian cellular turnover. They degrade polypeptides and are distinguished by their substrate specificities. Cathepsin D is the lysosomal aspartic proteinase, active in intracellular protein breakdown. Cathepsin D is involved in the pathogenesis of several diseases such as breast cancer and Alzheimer disease. The SensoLyte® 520 Cathepsin D Activity Assay Kit uses a QXL™ 520/HiLyte™ Fluor 488 labeled FRET peptide substrate for measurement of enzyme activity. In the intact FRET peptide, the fluorescence of HiLyte™ Fluor 488 is quenched by QXL™ 520. Upon cleavage of the FRET peptide by the active enzyme, the increase of fluorescence can be continuously monitored at excitation/emission = 490 nm/520 nm. The fluorescent signal from HiLyte™ Fluor 488 is stable at low pH, which is optimal for cathepsin activity. The kit can be used to detect the activity of Cathepsin D enzyme in biological samples and purified enzyme preparations.