Cathepsin B belongs to a family of lysosomal cysteine proteases known as the cysteine cathepsins and plays an important role in intracellular proteolysis. In humans, cathepsin B is encoded by the CTSB gene. Cathepsin B is upregulated in certain cancers, in pre-malignant lesions, and in various other pathological conditions. Cathepsin B may enhance the activity of other proteases, including matrix metalloproteinase, urokinase (serine protease urokinase plasminogen activator), and cathepsin D, and thus it has an essential position for the proteolysis of extracellular matrix components, intercellular communication disruption, and reduced protease inhibitor expression. It is also involved in autophagy and catabolism, which is advantageous in tumor malignancy, and it is possibly involved in specific immune resistance. Additionally, it was recently determined to have minor ligase activity with the ability to attach peptide fragments via an amide bond.