Cathepsin B, a member of the papain family of cysteine proteases, has also been called Cathepsin B1 and APP secretase. It is a ubiquitous lysosomal protease that functions in protein degradation and apoptosis. It possesses both endopeptidase and dipeptidyl carboxypeptidase activites. It can also be secreted from the cell and is implicated in disease states such as arthritis, gingivitis, and tumor metastasis. Cathepsin B is best used to study enzyme kinetics, cleave target substrates, and screen for inhibitors.