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 S is a cysteine proteinase involved in the pathogenesis of autoimmune diseases, atherosclerosis, cancer, obesity and related diseases. This peptide is a cathepsin S substrate fluorescently labeled with AMC (Ex/Em=354 nm/442 nm). It can be used to measure cathepsin S activity.