Cathepsin B (CatB) is an enzymatic protein belonging to the peptidase (or protease) families. It is the first described member of the family of lysosomal cysteine proteases. It is known to process a number of proteins, including pro and active caspases, prorenin and secretory leucoprotease inhibitor (SLPI). It is believed to participate in intracellular degradation and turnover of proteins. Cathepsin B may play a role in activation and inactivation of caspases, activation of renin and inactivation of SLPI, the key steps in apoptosis, angiotensin production, and progression of emphysema, respectively. Cathepsin B may also have a role in invasion and metastasis.