Cathepsins (Ancient Greek kata- "down" and hepsein "boil"; abbreviated CTS) are proteases (enzymes that degrade proteins) found in all animals as well as other organisms.

There are approximately a dozen members of this family, which are distinguished by their structure, catalytic mechanism, and which proteins they cleave[citation needed].

[10] Overexpression of the encoded protein, which is a member of the peptidase C1 family, has been associated with esophageal adenocarcinoma and other tumors.

Cathepsin D (an aspartyl protease) appears to cleave a variety of substrates such as fibronectin and laminin.

Cathepsin K is involved in osteoporosis, a disease in which a decrease in bone density causes an increased risk for fracture.

[14] The genetic knockout for cathepsin S and K in mice with atherosclerosis was shown to reduce the size of atherosclerotic lesions.

Cathepsin K inhibitors, Relacatib, Balicatib, and Odanacatib, were terminated during clinical trials at phases I, II, and III, respectively, owing to adverse side effects.

After electrophoresis, the gel is put into a renaturing buffer in order to return the cathepsins to their native conformation.

The earliest record of "cathepsin" found in the MEDLINE database (e.g., via PubMed) is from the Journal of Biological Chemistry in 1949.

Much of this earlier work was done in the laboratory of Max Bergmann, who spent the first several decades of the century defining these proteases.

[26] It is notable that research published in the 1930s (primarily by Bergmann) used the term "catheptic enzymes" to refer to a broad family of proteases that included papain, bromelin, and cathepsin itself.