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. Most of the members become activated at the low pH found in lysosomes. Thus, the activity of this family lies almost entirely within those organelles. There are, however, exceptions such as cathepsin K, which works extracellularly after secretion by osteoclasts in bone resorption. Cathepsins have a vital role in mammalian cellular turnover. Classification Cathepsin A (serine protease) Cathepsin B (cysteine protease) Cathepsin C (cysteine protease) Cathepsin D (aspartyl protease) Cathepsin E (aspartyl protease) Cathepsin F (cysteine proteinase) Cathepsin G (serine protease) Cathepsin H (cysteine protease) Cathepsin K (cysteine protease) Cathepsin L1 (cysteine protease) Cathepsin L2 (or V) (cysteine protease) Cathepsin O (cysteine protease) Cathepsin S (cysteine protease) Cathepsin W (cysteine proteinase) Cathepsin Z (or X) (cysteine protease) Clinical significance Cathepsins are involved in many physiological processes have been implicated in a number of human diseases. The cysteine cathepsins have attracted significant research effort as drug targets. Cancer, Cathepsin D is a mitogen and "it attenuates the anti-tumor immune response of decaying chemokines to inhibit the function of dendritic cells". Cathepsins B and L are involved in matrix degradation and cell invasion. Stroke Traumatic brain injury Alzheimer's disease Arthritis Ebola, Cathepsin B and to a lesser extent cathepsin L have been found to be necessary for the virus to enter host cells. COPD Chronic periodontitis Pancreatitis Several ocular disorders: keratoconus, retinal detachment, age-related macular degeneration, and glaucoma. Cathepsin A Deficiencies in this protein are linked to multiple forms of galactosialidosis. The cathepsin A activity in lysates of metastatic lesions of malignant melanoma is significantly higher than in primary focus lysates. Cathepsin A increased in muscles moderately affected by muscular dystrophy and denervating diseases. Cathepsin B Cathepsin B may function as a beta-secretase 1, cleaving amyloid precursor protein to produce amyloid beta. Overexpression of the encoded protein, which is a member of the peptidase C1 family, has been associated with esophageal adenocarcinoma and other tumors. Cathepsin B has also been implicated in the progression of various human tumors including ovarian cancer. Cathepsin D Cathepsin D (an aspartyl protease) appears to cleave a variety of substrates such as fibronectin and laminin. Unlike some of the other