Cathepsin K (Cat K, CTSK; EC 3.4.22.8) is a member of the CA1 family of lysosomal cysteine proteases and of the papain family. It is considered to be the major enzyme responsible for degradation of the organic bone matrix. It is highly and selectively expressed in osteoclasts and, under acidic conditions, has the unique ability to degrade type I collagen helical regions. Unlike the other cathepsins, Cat K not only degrades type I collagen in the telopeptide regions, but is capable of cleaving the triple helical domains at multiple sites. The protease is an attractive target for inhibition of bone resorption.