A cathepsin is a protease, a type of protein that breaks apart other proteins, found in many types of cells including those in all animals. There are approximately a dozen members of this family, which are distinguished by their structure 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.
Cathepsins have been implicated in:
- Alzheimer's disease,
- Ebola, Cathepsin L and to a lesser extent cathepsin B have been found to be necessary for the virus to enter host cells.
However, references within this article indicate that they were first identified and named around the turn of the 20th century. 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. 
- Nomura and Katunuma. Involvement of cathepsins in the invasion, metastasis and proliferation of cancer cells. J Med Invest. 2005 Feb;52(1-2):1-9. Review.
- Lipton. Ischemic cell death in brain neurons. Physiol Rev. 1999 Oct;79(4):1431-568.
- Pham. Immunity. 2005 Jun
- Maver and Greco. The hydrolysis of nucleoproteins by cathepsins from calf thymus. J Biol Chem. 1949 Dec;181(2):853-60.
- Bergmann and Fruton. Science 1936; 84(2169):89-90.
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