Apoptosis can be mediated by mechanisms other than the traditional caspase-mediated cleavage cascade. There is growing recognition that alternative proteolytic enzymes such as the lysosomal cathepsin proteases may initiate or propagate proapoptotic signals. Cathepsins are lysosomal enzymes that are also used as sensitive markers in various toxicological investigations. BioVision’s Cathepsin K ELISA kit is a sandwich ELISA assay for the quantitative measurement of CTSK in human serum, plasma and cell culture supernatants. The density of color is proportional to the amount of CTSK captured from the samples.