For example, cathepsin Bis secreted by cancer cells into the tumor microenvironment41−43 where cathepsin B activity can be monitored by Z-Nle-Lys-Arg-AMC.Furthermore, bacterial infection promoted cellular cathepsin B activityat a neutral pH environment61 and, thus,the Z-Nle-Lys-Arg-AMC can be valuable for monitoring cathepsin B atneutral pH cellular environments in infectious diseases. Here, CTSB is linked to cancer.